What about a usual improving while you are playing (or not) like many proposed here, so fighters for example, do not need to spend many hours to improve their smith skills BUT there is a bonus on the skill if you spend more time on this which can decrease if you do not practice your craft for a while.
This way, both kind of players can create great items but only the true "smiths" can excel at creating the best stuff.
Of course this should be balanced in a good way.