I'm not sure how Star Wars works but if it is at all like the Basic Role Playing system where characters make a roll at the end of every session to see if any skills they used are increased, and so the only way to increase skills is through rolling them, then I think the existence of Karma might create an inherent limitation on abusing the system. Because no matter how much you roll and reduce training time, you still have to spend Karma, so there's not a lot of utility in looking to roll all your low skills as often as possible.
But a possible solution to this potential abuse is to limit it to once per session. Not once per session per skill, but once per session at all. So the Street Samurai can't spend the whole session firing at the hardest to hit targets with his worst weapon and skill to claim it as training (and that's an obnoxious way to play a group game), he can only benefit from that once. And the Rigger can't keep speaking over the Face to make negotiation rolls to try and build that up, because if she does, she can't train her piloting rolls at all that session. Narratively, the once per session limit can be described as the character not just failing, but considering their failure and reflecting on it, which they can't do for every single misstep along the way. If the mage is thinking about why their spell didn't work, they're by exclusion not thinking about what they did wrong when they were sneaking.
