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I have noticed that some people just don't seem to understand the intricacies of traversing isometric tiles in an effort of escape. The purpose of this guide is to inform the reader how to flee from player-combat. Playing a thief (and poor pvper), you quickly learn that the best way to win a losing battle is through honorable retreat!
(Note: This guide also acts as the second part of Guerrilla's PvP Manual)
Before you jump off into the exciting field of retreatery, you may want to check out my guide on "how not to die", to get a better idea of what you should be carrying.
The Basics
Ultimately, the most common place that you will commit your noble act of flight, is the forest. The developers of Ultima Online were apparently quite lazy, and instead of painstakingly crafting the location of each rock and tree, they used some sort of algorithm to handle the task for them. As a result, YOU, the runner, benefit. It would be quite annoying if this coded tree-placement created a forest so dense that you could not walk through, fortunately that is not the case. The base pattern of the forest seems to place the trees with at least a 1 tile buffer between the other trees. Perhaps afterwards, it went back through and placed a few random other trees and moved some to vary the pattern a bit.
As the following image demonstrates, the forests usually have a few natural pathways that you can travel without meeting an obstacle.

The light green pathway represents the four cardinal directions. There are places in the forest where you can follow a grid through the trees and not worry about blockage, save for the occasional pig or other npc. The most efficient way to traverse the forest is to follow these exact pathways, only taking a single diagonal step to change your direction (when possible).
If you read the recommended reading listed at the beginning of this guide, you know that one of the most helpful items to have in your possession is a common table (or footstool, it really doesn't matter too much). If you are being pursued, aligning yourself to this foresty-grid and dropping a table can put a significant distance between yourself and the attacker(s). Try to drop your blocking objects in a location where the follower will have to take more than 2 steps to get back on course. For instance, the intersection of the leftmost light-green grid in the image. The trees will work to your advantage.
Alternatively, you can make your attackers block themselves. For the purpose of this discussion, let's say that you are heading North through the grid with an attacker following close behind. If you followed the left-most light green trail to the north, you would see a darker green pathway moving off to the Northeast. Depending on various things (lag, mount, etc), your follower is usually about 5-10 tiles behind you. When you are running you always have the advantage, because only you know where you are going to go. You can use this to your advantage by making a sharp diagonal turn when you go behind a treebush or other tall object (houses etc). If you see a clearing ahead and have the opportunity, cut along the diagonal and run until you align yourself with a parallel forest grid, then keep heading in a direction.
Since your follower usually lags behind a few tiles, there is a good chance that they will cut over as well. However, since they did not see the clearing up ahead, they will cut sooner down the trail than you, and likely end up hitting a tree, or, at the very least, adding several precious extra tiles to their trip. If you were to cut along the dark green tiles, your attacker would likely cut over around point A in the image. It is not possible for them to efficiently make their way to the next forest-grid without hitting the tree at point B and adding tiles to their journey.
What if there is an NPC blocking my way?
Suppose you are running along the grid and notice an animal blocking your path, or worse, a monster (represented by the rabid canadian feline in the image). Every hostile monster in the game has a certain looping cycle it uses to determine when/who it attacks. Unless it's something that is really intelligent like a Balron or a Kaivan, you can probably run right past it without taking any damage. If you are lucky, it will lock on and deal some damage to your attacker.

Monster or not, the protocol in the situation should be the same (unless it is a balron/EV etc). While you are running you should be watching your stamina and keeping a finger on three important hotkeys. One of these hotkeys is "Drink Refresh Potion". If you drink one only moments before you run into the side of this canadian devil, you can push through, and shave a couple steps off your time.
The second of the important hotkeys to keep ready, is the teleport spell. The teleport spell has two very practical applications. The first one, is obviously to teleport. Teleport is a relatively low circle spell, and is very difficult to interrupt. If you cast the spell and run towards a location with a barrier such as a fence, you can easily teleport over and out of range of the attacker, who will have to cast the spell himself, or fumble to use a teleport item. As soon as you teleport, get offscreen and cast recall. If you have teleport items yourself, you can just teleport 3 or 4 times in the direction you are running and immediately hit your recall key. They will have no chance of catching you because they would have to teleport to keep up, which means they have to drop the harm they have had precast to disturb your recall. Occasionally, there is no barrier nearby to teleport over, but the spell can still work you your advantage. Most of the time, when you cast the spell, the attacker will cast it as well. This gives you a great advantage. When you cast, watch your attacker, and as soon as they cast teleport, cancel yours and press your recall macro. It would take them at least 1 second to re-arm their weapon, and now you know that they don't have harm up to disturb you. Furthermore, they will have a delay after the teleport spell, before they can even attempt to recast harm. Teleport FTW
The third of the important hotkeys is the good old recall macro. Record a razor macro to cast recall and target a rune by absolute target. Here is what you need to know about it.
Set the macro to a hotkey.
Do not use target by type.
Set the absolute target when you log in!
Do not panic and spam the macro.
Mark the rune in a NO-TRAFFIC area.
Always have a second rune just incase.
PK's will chase you for as long as it takes, casting harm on your recalls the entire way. If you spam the macro over and over, you set the tempo for their attack. They will hit you with harm and a halberd in a predictable pattern. Cast this macro first thing, otherwise take off running and cast it at an opportune time. One other trick that you can play with this, is to arm a magic-reflect item before casting to block their disruption, keep that in mind when stocking your backpack.
The fourth important hotkey is the "use trapped pouch" macro (What? Oh yes I know I said there were three, but this one is extremely important as well). The ever growing trend of t2a-accurate para-spam has made it increasingly important to carry more and more of these helpful little items. Learn how to set up the use-once agent in razor, and bind these to a key. When groups of pk's (or just pvpers) attack, one of them is always doing the damage, the other is casting harm to keep you from escaping, and if there is a third, you can bet he is spamming you with para while the others hit you. The para spell takes 14 mana on each use. One person alone can para you 7+ times on a full tank of mana. For that reason, I always carry a minimum of 8 pouches.
Ok, so I am running, now what?
Now try to escape! Drop objects to make distance to recall, teleport over nearby barriers (or just use the teleport-juke mentioned above), use MR items to block disrupts, head for the nearest guard-zone. If you are in the forest, the best possible direction to run is to the west (up-left).
Why to the west??
Because there is so much complexity in the ancient art of uo-pvp, many players have in-reach macros set to only the critical spells of leet red-pots-only omgbbq pvp, as dictated by the age-old honor-code of t2a uo pvp (age-old honor-code of t2a uo pvp is a registered trademark of Faust LLC.) Sometimes, critical spells, such as teleport (and para even) do not make it into this grouping of common macro keys. Some people use those little tile things you drag from your spellbook!
Anyone who has played the game for more than 10 minutes, knows that in order to run, your mouse has to be in the direction you want to move. For some reason, practically everyone in the history of UO puts their little spell tiles on the right side. If they have no para (or harm!) macro bound (ok, everyone has a harm macro) they will have to click the little icon, and to do so, they have to stop for a second. They can't follow you, and click a tile at the same time if you are heading to the west.
Even my spells are on that side!

Sometimes there is just no room to run
Let's face it, sometimes people don't want to kill you in the forest. Sometimes they want to kill you in town, at the Terrathan Keep, on the Bloody Plains. Fortunately, the most fundamental idea of running still comes into play. It's all about making the most efficient moves.
Hiding
If you are in a dense housing area, you can always try to hide near a house. Even if you have 0 hiding, there is a pretty good chance that you can hide right up against a player-house. There is one important thing to note about this. NEVER hide when the attacker is on-screen. If you do that, all you did was give him time to get near you with a purple potion. Recalling while they reveal may seem like a good idea, but only newbie pk's cast that.
Stealth
Cast invisible, or use an invis item and immediately stealth away at an angle. You do not want to follow the UO cardinal directions when you make your stealth getaway. You can put a greater distance between yourself and the hunter if you use an angular movement.
Doors
Doors can be your best friend. If you are in a location with a door, run over to it and pop it open but do not go in. Run around a bit longer so you are sure your one-second action delay has elapsed, and then run through pressing last object to pop the door closed behind you. Promptly hit your recall macro.
Wall of Stone
This is a truly fantastic spell. The obvious use is to set a critical choke point as last target, (You do have a set-last-target hotkey don't you), and then run through and drop the wall, at which point you can recall or whatever. Most of the small houses on the server are unlocked! Alternatively, you can just cast it, but hold the cursor long enough to fast-cast the next spell, and then drop it and recall. If you drop a wall of stone in the right area, you can cause your opponent to have to cast dispell, or have to take another 3-4 steps to get LOS on you again. Since nobody can see your text without first having LOS, you can always use it to prep other spells that people are not expecting either, EQ, Meteor, etc.
Private Houses
Run into a complicated private house. Towers and such tend to work best for this. Run all the way into the back of the dwelling, hide, and log out. Then immediately log back in. You should now be outside, cast recall. This works if you don't have hiding, but you had better be really quick about it.
That's all I can think of for now, but I know I am missing some things!