How can I get frozen egg whites out of the ice tray? I read in Can raw eggs be frozen? that you can freeze eeg whites and use them later.  

I saw this suggestion about using an ice tray to make frozen egg white cubes (which makes it easier later on when you want to use a few eeg whites out of a frozen batch).  

My problem is, the frozen cubes won't come out of the ice tray! They seem to expand or for some reason stick to the tray very hard. I needed to melt them by running the back of the tray under hot water to get them out.

Obviously I can't use any oil or anything like that in the tray to prevent sticking.
Any suggestions?
So to sum up, so far, we have:  


use a silicon tray, there should be no problems there.
put the tray in warm water for a short while and they would let go.