Strange but mostly true...
1) Your fingernails grow faster when you practice a lot
...Especially the fingers you use most often. French horn? Thumb, index, middle, and ring on the left hand!
2) The more you practice, the worse you are at sight-reading
This doesn't seem correct until you realize that before you started practicing regularly, most of the playing you were doing was sight-reading. As you practice, you are exercising the part of your brain that is responsible for remembering how things should feel and sound, not how to see something and take your best stab at it! To counteract this, practice scales and arpeggios every day and also keep your sight-reading sharp by reading though something you're not familiar with daily or every other day
3) Practicing is more tiring than almost any other type of playing
...Because you don't stop for the conductor to talk to the strings... or for the winds to tune something up... or for the violinist's cadenza. And you repeat passages more than you would in performance. For that reason, you should build breaks - short or long - in to your practice. So maybe you practice for fifteen minutes and then take a five minute break or practice for 45 minutes and then break for fifteen. In any case, you'll give your face - AND YOUR BRAIN! - a much needed rest.
4) What you work on really hard today might not sound better until tomorrow
We've all been there. You work painstakingly on a phrase, slowing it down, trying to get the right kind of ease. It doesn't work. It's still difficult and clunky. Or you are looking for a type of sound or dynamics and you just can't hit it - it seems impossible! But then, you wake up the next morning and like magic, it's there! If something isn't working in the moment, don't despair. Know that the thoughtful work you put in will make a difference; you just might have to sleep on it! Speaking of sleep...
5) You can practice in your sleep
Well, not exactly. But your brain continues to process problems you present it with during the day while you slumber. For that reason, it can be especially helpful to visualize playing something you're working on while you're in bed, about to go to sleep. Always try to visualize a performance that's going really well. You might even realize by doing this that your brain is messing you up when you're playing irl.
6) Spinning around can help you learn something
The next time you are working on a lick that you can play but you're worried you can't play it all the time (*cough* at your lesson *cough*) try this: Play the lick facing in all directions of your practice space. Having your brain adjust to doing the same job but while taking in different scenery can help you be more consistent! This also works in a classroom setting. Students who sit in different seats throughout the semester learn and retain more than students who always sit in the same place.
7) Practice DOESN'T make perfect
You may have heard this one before. Practice makes PERMANENT. So if you've played something wrong six times and right one time, chances are you will keep playing wrong. To make sure you've really got it, play something right at least 4 times in a row.
8) Fundamentals are for everyone
Let's say you "work out" a fundamentals issue with your teacher. You're really proud the day when you can feel like you've mastered the skill. Hurrah! You're not using "twah-twah" air or letting your tongue get in the way of your air! You can move on with your life now, right? In actuality, professional players are working out fundamental issues all the time, until the end of their careers. Your teacher? Still working on fundamentals. The principal player in a major philharmonic? Still working on fundamentals.
9) Listening can be better than playing
Have you ever worked hard on an excerpt or piece and then finally heard it played and realized you learned it all wrong? This can be especially hazardous for orchestral or band excerpts when we don't see the whole part or score in front of us. We might not see the tempo marking or understand the texture of what we are meant to play along with. We can save tons of time by just listening to the piece before we begin learning it. Preferably, we listen while looking at a score (all the parts on one page.) Another helpful tool is to record yourself during your practice and listen back. You might be surprised by what you hear!
10) Practicing can be fun
What now? Yes, indeed, even YOU can have a good time while practicing hard to master your instrument. A lot of this depends on your personality. Maybe you're the kind of person who doesn't need to have fun while practicing. But maybe you're the kind of person who loves solving puzzles. So make each thing you do about solving. Or maybe you just like the feeling of accomplishment? So make sure you give yourself a cheer or gold star when you finally get something right! Or maybe you just like being around people. Set up a practice date with somebody! That way, you can give each other encouragement and props!
Happy Practicing!