Tell yourself "Okay, I will just work on such-and-such a piece for 10 minutes and my other piece for 5 minutes". It sounds pathetic, but generally by the time you've played that long, you are 'into it' again.
It also helps if you pick a couple places that need 'spot work' because usually 5 or 10 minutes doesn't end up being enough, and before you know it, 45 minutes have gone by.
Remember, 15 minutes of practice in which you really accomplish something is better than forcing yourself to do 3 hours of mindless diddling!
Wow, I think I'll go practice now!