Keep your trainer fully paid (“happy”)
Pay larger amounts at once rather than small amounts frequently
Killing easy, numerous crits gives better skill than harder, scarce ones — if you can 1-2 shot crits, you kill more in the time it takes to kill one hard one
Hally users: skill where you can use 2-step or 4-step “dance” tactics — avoid melee damage
Same applies to ranged users (bow, crossbow, mentalist, healer) — avoid melee
For melee weapons: wear best defense or skill in easier places
Thief skilling: Steal from N-1 or N-3.5 crits with trainer paid to reach skill 12 easily. Don't steal from players.
Ment/Healer skilling: Form disciplines on blue-tagged NPCs (like your trainer) repeatedly. Good for lower skill levels.
N-3.5 ghost is good skill up to 8; Ogre Chief up to 12
Aleria is good for skill above 11
Partying with a thief boosts your skill (as long as the thief uses backstab, steal, etc.)