As a player, I'm self-taught, and have also learned a bit of repairing by tinkering around with things (fixing leaks and general adjustments like spring tensioning etc). My advice from this standpoint is to be firm yet gentle, don't overdo things, and remember where you started from when you are attempting things like screw adjustments, as unless you examine your horn quite carefully to see what effect you are having by, say, adjusting a certain screw you can fix one problem and cause another one somewhere else unexpected. On the other hand, I'd say don't be afraid to experiment, as you always have your local friendly repair man, hopefully.
Cheers, Bob