Before you start painting with a roller, try this and I swear you'll never do it any other way again!
Get a plastic bag (no holes) big enough to fit over your paint tray in with a bit extra so it fills the recess where the paint goes too. Use some masking tape to seal the bag with the tray inside. If you're using a mini roller tray a sandwich bag may fit best.
Use the tray as normal. When you've done, use a brush to squidge any remaining paint back into the tin. Remove the tape, put your hand inside the bag and grab the painty bit and turn it inside out. Put it in the bin (or put your roller in it if you want to start painting again the next day as the bag will stop it from drying out.)
I do this every time I paint and it's so nice to not have to clean out the tray as well!