The most important thing is to Stretch! Take short breaks while practicing to walk around, swing your arms in full circles, do your favorite stretches or just move around and put yourself in a different position for a little while. It doesn’t seem like a big deal but it's one of the best things you can do for yourself, really! If you feel uncomfortable, are in pain or have numbness, get help for whatever hurts and advice on changing how you stand, sit or hold your instrument and make sure you do it right away. NEVER ignore pain or numbness--that’s what leads to permanent injuries. It may take several tries to find the position that lets you play properly without discomfort but don’t give up. Ask for help from others if the first person you speak to can’t help you. The most common time to develop repetitive use injuries is when you are abruptly increasing the amount of time you play such as dress rehearsals for a show or the first year or two of college or graduate school. This is also the time musicians are least willing to cut back on playing just because we have a twinge somewhere. We tend to make things worse by ignoring whatever the issue is "just until this next show is done”. Don’t! Get help right away.
My personal story--In Grad school, I developed some mild discomfort in my shoulder that came and went but never completely stopped nagging me. Even at its worst, it wasn’t that bad but it was fairly constant. It bothered me while doing things other than playing (like driving) and often bothered me while I was trying to go to sleep. I tried stretching for a week or two which helped short term but the discomfort always came back just as bad as before. Sometimes it seemed tender or sore, but mostly it just felt odd. Tingly but not really numb. These are several BIG warning signs that I recognized from other musicians’ stories. They are also EARLY warning signs that are easy to ignore but I knew that would be a big mistake. I immediately tracked down a chiropractor who specializes in repetitive use injuries that people such as sports players, musicians and artists often experience. A sculptor who spends hours pounding chisels and hammers into rock sent me to him. If he can help a sculptor, he knows what he's doing! He uses a technique called active release which is basically a localized intensive massage combined with manipulating the joint meant to put everything back in working order and reduce inflammation and pain as quickly as possible. He also gives his patients exercises to do at home so that they will eventually NOT need to see him at all and can take care of things on their own (the sign of a good chiropractor). After a few visits, I felt wonderful, that is, normal. Once or twice over the next few months, the issue flared up a little but not as bad as before and I went in for a “quick fix”. As time went on (and I got better at remembering to do the exercises) the exercises made more and more of a difference and made my shoulder feel better faster and faster. Now, if I forget to do the exercises for long enough (more than a month or two), my shoulder may start to bother me again but as soon as I do the exercises, I’m fine. And it takes longer and longer for the issue to return the more I do the exercises.
The moral is, don’t pretend things will get better on their own. The sooner these types of issues are dealt with, the easier they are to get under control. It may not be that bad to start with but waiting will make it worse. Don’t accept “you’ll have to live with it” for an answer!