This guide talks about this:
Understanding the Arts But the short version: Basicly, skillups are tied to applying your tradeskill arts. You really need to be using white-con arts (usually your most recent) to be assured of regular skillups. You can get skillups on grey arts (or no arts at all) but eventually you'll fall well behind the level cap. If you prefer to use other arts regularly, just use the newer arts long enough at the start of each level to get your 5 points - usually no more than 5 combines, often much less. If your skill has fallen way behind your cap (which will definitely cause problems for you) do this:
- Pick a really cheap recipe - like T1 coffee
- Find your most current, white-con arts
- With complete disregard for the results, start the creation process and spam the ever-lovin'-heck out of those white con arts. You may fail the combine, which is why we picked T1 coffee. The more you hit those white-con arts, the better chance you have of skillups.
- When your skill hits the cap, mourn the 1s you've spent getting it there, sell off whatever's left over from the process and go back to crafting the way you like.
- Next time you ding a level, make sure you're incorporating at least one white-con art into your crafting routine, and you shouldn't have a problem keeping that skill up.