From elpool:

Don;t even blackjack AI guards if you don't have to. When a human guard places an AI guard in a key spot, he will become more relaxed in that area, assuming the AI is there to take care of things, and will be less likely to search the area. also, many guards (like myself) try to remember the names of the AI and where they are placed, so that if he gets KO'd, human guards will come running to investigate. also, if you kill off all the guard team leaving only 3 or 4 human guards, they will become more entrenched, and never stop camping objectives.

For getting away from Guards when they notice you, just use flashbombs or invisible potions (or a catfall potion if your on a roof) and run as far away as possible. try to find shadows in the levels that guards don't usually check and use them. sometimes there are shadows that will completely conceal you, but don't LOOK dark enough to hide in. also, jumping up on tables or crates helps, guards are less likely to check there.
 

From BrokenArts:

The more you play, the more tricks you will learn.

For thieves.

1. Moss arrows can cover caltrops.

2. Lockpicks are handy for disabling and picking up mines.

3. Learn to hug the walls, shadows are your friends, some area's that look like you should be visible, example Breakout, hugging the walls along the stairs, and along the pillars, before the library area, some spots, your invisi.

4. Don't always put out so many torches, the guards know you have been there, or are still there, again, find those shadows.

5. Learn to work together as a team sometime with other thieves, it comes in handy for tough spots. One can be a diversion, while the other does his/her thing.

6. Also team work comes in handy for knocking out human AI.

7. Catfall potions are fun, ha, comes in handy for getting away, when you have to jump off pool edges, like in Spider, jumping over rails, in Grange, and Breakout. They are cheap to buy, and loads of fun!

8. Try to access the entry points from a from all angles, if you need something, and they know it, and they are there waiting, you tried one door, go try another, try another way in, there is usally more than one entry point.

9. You can open doors, and sometimes fool them into thinking you are coming in, and when you don't, *scratches head* no one there, ( a ruse) sooner or later, you'll be in.

10. Creep and BE QUIET.

11. Use arrows for regular AI if you want them off a spot, if they are facing you, and you need them to move, sort of a noise/footstep arrow, shoot in the direction you want them to go, they'll start their search, and you can black jack them, or sneak on by, choice is yours.

The more you thief, you will become better and better, slowly, and ghost more too, that is the Ultimate.
 

From Neopendragon:

Leave NO evidence! If human guards know you've been to a room, they usually become more careful and search the area immediately. What you have to do is, close doors behind you (unless they were already opened), close chests after you open them, and don't douse torches unless its absolutely necessary. If you can hide in a room without dousing torches, that's great because they won't suspect a thing. Same goes with AI guards, if you can sneak by them without knocking them out, the human guards will have no idea where you are or where you've been. I've found that that can be a bit more dangerous though, and you can find yourself surrounded if someone finds you. And when there's a smaller number of human guards in a match, I tend to do hit and run raids. If guards are camping the Dining Room and Painting Gallery in Nostalgia, for example, I'll clear the rest of the level and by that time they sometimes try to hunt me down from boredom or nervousness or whatever and then I hit the room when they go. And if they don't go and you're playing with another thief, one of you can be a distraction and have them get chased out of the room into another area. Then that's when you yoink the treasure and run.
 

From Seymour_Gibbs:

Don't Run away! Running makes you visible which is a bad thing. Its better just to run to the nearest shadow, then crouch and crawl to safety. The guard will shoot at where you were last seen.

Be patient when blackjacking - a charged swing always KOs, but its got to be to the back of the head. Wait while he's past before stepping out.

Don't get holed up - keep moving towards objectives or you'll be out of time and surrounded by caltrops.

Close doors behind you - this goes for guards too. An open door is like a pointer to your position.
 

From LaughingRat:

A lot of what NeoP said boils down to: Teamplay. Use the whisper to coordinate and plan with your teammates, whether you're a guard or thief. If you're a thief, one of you be a distraction to draw guards away from an objective so another can grab it. If you're a guard, and one of your teammates seems to be chasing a thief, back him up (unless you're leaving a key objective unguarded by doing that). One guard alone can be easy pickings for a skilled thief.

I've found that hit-and-runs work well. Get into a building and clear out the immediate area, but don't try to go all the way through. Back out, and enter from another point. Not possible on all maps, but works well on Grange, Gerome, Flats, and Bourgeois.

But most important, teamplay. If your teammates make mistakes, don't bitch. We all do that, and you degrade your ability to work together by doing that. And above all, remember, it's only a game. You're not going to win every round, so don't get upset when you lose. Just congratulate the winner, and play the next round.