While repetitive practice is not always enjoyable, it's productive and it's good for you in the long run. Many music students find their practice sessions frustrating and unproductive all because A) they're practicing the section of the music at performance tempo (which is way too fast for "practice time") or B) they're trying to tackle a large chuck of the music rather than honing on one particular small segment of the composition. Slow repetitive practice with a metronome is extremely helpful when faced with difficult passages in music.
Another thing to keep in mind when you practice is don't overdo your practice time. That's one of the biggest problems I face ALL THE TIME. Some days, I'm overwhelmed by the amount of music I need to practice for this concert and that recital and this wind ensemble placement audition that I find myself becoming a musical hermit who has locked herself into a practice room for half of the day. DON'T DO THAT! Short productive practice sessions are definitely more helpful than long, intensive, and painful torture sessions. And that brings me to another point. If your practicing starts to bring pain, STOP! Pain is the red flad for you to stop with what you're doing and give it a rest. Don't work through the pain. Just relax for a while and get back to practicing when you're feeling better. Use the "fifteen minute" rule. Practice for fifteen minutes, then relax for fifteen minutes, then practice for another fifteen minutes, and repeat this process for however long you desire. Overpracticing can definitely lead to some injuries if you overdo things like me. Muscle knots, carpal tunnel, aching fingers, bleeding stumps of fingers, purple/dark gray fingertips...I've experienced all of that and it's not fun and definitely not good for you. Eight hours of straight practice is not worth getting carpal tunnel. Leave the eight hours of practice for those Juilliard students.
When you practice, you should not just practice until you get a rhythm or passage correct. Practice until you can't play anything wrong. This mentality makes for a better musician and a more professional player. Also, try to practice every day. I try to cram at least an hour of practice in each day, but occasionally, that just doesn't happen. Please don't be the middle school band slacker and practice only two times a year: the night before your band audition and the day of your spring concert. Take the instrument home and practice. Work simple exercises such as scales or lip slurs. Working on technique won't kill you. It'll end up making you a better player if you have good practice sessions.
So my words to you, reader, are go out and practice an instrument! If you don't play an instrument, go pick out one and start practicing! Practice will hopefully make perfect.