My lyrics are always written on the bathroom wall! That's where I get them from, that's where they ought to stay. (Sorry, I couldn't resist).
Seriously, when I forget I usually find myself rhyming backwards. I'll remember part of the verse, then I rhyme backwards to the beginning. If I have the beginning of the verse, I can almost always remember the rest. If not, I make stuff up, 'cause I'm really screwed.