In the summer months, you might notice that you have a hard time keeping your lip plate stable against your chin, since our faces get sweaty when it's hot out. The lip plate slides around when your chin is slippery, and makes it hard to get a focused sound.
I've heard a bunch of remedies to this problem, ranging from sticking a postage stamp on your lip plate (inadvisable) to applying antiperspirant to your chin (probably bad for your face and your flute's finish). One trick I use when it's humid out is to place a small piece of paper medical tape on my lip plate where it will come in contact with my chin, like so:
The tape creates a little friction between your chin and the flute, preventing the sliding-around that tends to happen when our faces get sweaty.
It's important to use a tape with a gentle adhesive when you do this: I like Nexcare, but any medical tape that bills itself as "gentle" or "low-adhesive" works just fine. This will prevent any damage to the finish of your flute.
Another thing you must do if you use medical tape on your headjoint is remove it when you pack your flute up at the end of the day. Keep a roll in your flute bag and apply a new piece every day, but never leave it on overnight. This will help prevent buildup of adhesive residue and will also keep you from getting breakouts due to sweaty, old tape touching your face.
So don't let the dog days of summer get in the way of your practice routine. Tape up and good luck!