Musicians and vocalists are prone to overuse injuries due to the amount of playing and singing that they do. These injuries can have a devastating effect on the careers of performers.
See the page on repetitive strain injury re. general principles on how to deal with these.
I generally play the piano, but reduce the amount of repetition and physical stress by applying the principles of cross training by varying my piano playing with a bit of guitar and harmonica. In a way it’s not ideal because it’s probably better to focus on one instrument, but there are other advantages with being a polyinstrumentalist. I think that you learn more because if you play something on different instruments you look at what you are playing from different perspectives. The guitar and the harmonica are also very portable which is great. I’ve found the harmonica a particularly useful instrument to be able to play because not many people can play it and it works well with guitars.
Another way to apply the principle of cross training to playing a musical instrument is by having more than one piece that you are working on at the same time.This ensures that you are not repeating the same movements too much. To get the crosstraining benefit of playing the guitar and piano, you must ensure that you are not repeating similar movements on the guitar and piano e.g. if you are using your little finger a lot playing a piece on the guitar, avoid doing this on the piece that you are playing on the piano.
If you are reducing the amount of play you are doing in order to rest, then you may gain as much benefit from your practice if you practice in a morel concentrated way i.e. by doing less random playing.
All musicians need to be careful with their bodies whilst not playing. For example, it is foolish to get into any fist fights with people because this can damage your hands (and is not sensible anyway!). Look after your ears by purchasing specially moulded ear plugs if you go to gigs. Take breaks from the noise at gigs and in clubs. Follow the other advice about protecting your hearing that you can find online.