I tend to use underscores on join tables so table foo_bar would have a fooid and a barid. I have somewhat soured on this approach though since there are a lot of situations where you'll have two m-m relationships between the same two tables with a different meaning... and having a fixed formula for m-m tables can make things ugly.

If I get to design another greenfield database I'll probably prefer using underscores for word boundaries in long table names.