Hi Topman
Are you referring to emulsion paint ? If you are, then the easiest way is to let it dry completely over a few days and then just scrape it out in lumps !! (Presume you are using plastic tray). If you've been using gloss or u/coat, then wait a lot longer before you try and scrape. Always worked for me over the last 20 or so years.
Good luck
FBG40
Put a good quality plastic bag over your paintray before you fill it. After use throw the plastic bag away. This is also ideal if you want to paint with more than one colour with the same tray.