Well, this happens to me all the time and I always have to throw the tray in the trash!
But I found the solution to this problem!! While strolling though the paint isle of LOWES a couple days ago, I saw this paint tray that had Teflon lining.
Shur-Line Teflon Paint Tray
So, Of course, I decided to try it.
OMG!!!
It's amazing!!
After the paint dried,
it just peeled right off!! 
What an investment!!
You gotta get this!!!
