This is my weak attempt to track what has and what hasn't worked for thieves that I have played. If you have any suggestions, email or IM me.

• Master all relevant skills.
  • Blackjack has three rounds of lag. You don't want to miss against a PK. Three rounds vs. a shaman will get you blinded, cursed, and faerie fired. If you miss your blackjack, run.
  • Envenom: don't kill yourself trying to master it but make sure your weapons and daggers are coated.
• Get gold. You will never have enough gold. You burn through it quick on potions and emergency cures. You will live and die by your ability to find/use potions and healers.
• Thief tactics vary according to your style and the skill of your opponent.
  • Against a target that I think I can catch, I will open with backstab. Then try to come back with blackjack or throw. I haven't tried it with blackjack but throw was nice.
  • Anyone who I think has some skills gets blackjacked right away.
    • Gag if worried about casting or Justices.
    • Steal anything you need.
    • Steal recall potions and vials.
    • Pry weapons (no parry), shields (no shieldblock), beads, HP gear (against MUs).
    • Backstab.
  • Gagged mages can easily be out damaged. Avoid lagging yourself unless your trips are stopping them. Be ready to chase.
  • Warriors types with low to average HP races: if you take their weapons and land a solid backstab, you should be able to beat them. You will take damage but the backstab will give you a huge head start.
  • Dwarven, duergar, giant, and ogre warriors: you will have to rely on dodge to save your butt here. They will dirt you and disarm you. Use poison to weaken them. Hopefully you can trip lock them and finish them off. But the second it starts to go bad, run.
  • Dark-knights: drows and avians can be done in with one dual backstab at lower levels. But it should be a relatively easy fight for any prepared thief vs. a gagged dark knight who has suddenly found some of his gear gone.
  • Thieves and ninjas: this seems to be a situation where the first one to strike successfully will win.
  • Druids: you are a thief, don't attack them in the forest. Catch them in town. Blackjack them. Use detect magic and see if you can let their spells fall. They are too strong for a one hit kill, but if you can get them without their circles and shield of thorns, you will kill them.
  • Communers and psionicists: gag will only prevent their cries for help. You have to hurt them fast. Try to let their spells fall and uses your biggest attacks and be ready to chase or triplock them.
  • Bards: heh.
  • Paladins: a true pain in the ass. Take their weapons and shield. Dirt them (they don't have blind fighting). Then go for the lag. Daggers, daggers, daggers.
  • Necro: Blackjack, gag (like every other MU), avoid prying the weapon. Pry any HP gear. Go for the backstab. He is spamming flee so be ready to flee and chase him.
  • BMG: Gag them and steal any weapon that they might be using to dance (don't know if this works but if it isn't nodrop there should be no problem), pry their weapon. If they are brave, they are planning on their blades to finish you quickly. be ready to chase but also be ready to throw.
  • Monks: Try to get them between stances or in the offense (no counter or mantis). Pry their beads and steal any and all potions. They still might kill you though.
• On the run
  • Other than saves, there is no defense against faerie fog. Do not stay in the room with someone who will faerie fog you.
  • If you are faerie fogged, run like a madman. Now you must E and E like a weak warrior.
  • Poison can be cured by scrolls, plague has no cure that I know of except the healer. Always keep some gold in the bank for occasions like that.
  • Other area attacks can be avoided. Just sit be really powerful mobs.
  • Learn other places to hide than town. Ninjas and other thieves can be coerced into giving your enemies your location. Half the time they are the ones after you.
• Other
  • Dual backstab is not a one hit kill against anyone except mages and other low HP race/class combos. Be ready for them to try to go toe to toe.  Your best bet will be hit and run.
  • Use every potion that you can.
  • Pay attention to when you blackjacked your prey. Time flies when you are running around. Don't be afraid to blackjack them again just to buy you time to flee.