Easy to Remember
Short
Can be spelled as heard (I'm not going to say to someone, Hey my site is Digg.com, visit it. Oh yeah, we have 2 g's in the URL).
I usually get a list of them, and whichever I can remember the next day/few days without looking at the list again make it into the 2nd running, then whichever's shorter/with the best keywords/sounds best.