Skilling your ability

Solo
How much your skill increases is based on a kill per round. so if you are skilling your mentalist you will get more skill using a single target spell and killing slowly, then useing an Area effect spell and killing all at once. If you are skilling your thief skill, all you need to do is try to steal once, then kill.
I steal 5 times, then kill. But that is just my ocd kicking in and lay over from how it use to be.

Group
It doesn't matter if your attacks hit, you are getting skill off any party member kills. so as long as you are trying to use the skill that you want, even if you are "Out of range", you get skill.
As you are in a group the best thing for you to do is stay safe. You get no skill if you are dead. And you are taking skill away from the person who has to res (Cure Critical) you.

Area's
Mostly you are looking for an area that you can survive in, with a steady flow of mobs.
my thoughts are max skill in Nork is 30, but is very slow. 1% over 3 hours.
Max skill in Cob is 30, good places are UD and Pillers.
Max skill in Crazy Aleria is 35, good places are Tarak or Seldari Queen
Max skill in Nameless is depent on the area. A good spot for skill 30 to 35 is gdh first area. Generaly people seem to skill to 38 in thumpers, up to 40 in gnomes and then up to 42 in SK
You will want to be atleast skill 40 for Refuge otherwise you wont be able to hit much.