Well, just as a case study, in UO skills work by a variation of method A. You use the skill over and over to increase it. The harder the item, the greater chance of failure, and the more skill is gained with a success.
Another thing about UO skill system, they used to have a \"power hour\" in which you gained faster in the first hour you were on each day. This helped out people that played the game more casually.
Later, the moved to a \"Gaurenteed Gains System\" where each failure of a skill increases the chances of a success. Thus, you are \"gaurenteed\" to gain in the skill after so many uses. Again this helps the casual player.
IMO, I think both methods are nice, but kind of miss the target of the crafter. I\'d like something a little more involved in the world. Perhaps a combination of several methods.
Here are some ideas that I posted in earlier threads:
1) Start off with a variation of (A), but focus on making new items and creating unique items to gain in skill. People who always make the same simple item should primarily get money as compensation and gain little skill.
2) Complex materials/crafting system for creating diverse items. Exotic elements and items can create more skill (in crafting). Maybe a \"method\" system to help diversify farther. I.E. when making a sword, you choose to fold or simply draw the blade. Constantly changing methods increases skill.
3) Craft Skill Questing, you are given a quest to make a special item (must find instructions, materials, etc) or large number of items (like UO BODs). You get a bonus of skill (rationalized by teaching) for completing the quest.
4) The use of \"recipes\", where by you get a scroll that allows you to construct special items, gaining in skill. The scrolls are artifacts, but are of no use to anyone besides a type crafter. This increases trade between adventurers and crafters.