Like the others, I often flip mine over and tap the edge on the desk to get any debris out. I'll also wipe off the top with a wet wipe, and I'll insert the edge of the wet wipe (sometimes I have to fold in half to get a good edge) and run that between all the keys.
I tend to get a lot of hair caught in mine. I'm always amazed at the amount of hair (mine) and strangely enough, eye lashes and eye-brow hair!! You'd think I was bald.
A few months ago a co-worker asked me to show him how to do something on his computer. I sat at his desk with fingers poised above his keyboard and stopped dead. It was SO DISGUSTING! I told him to either replace it or clean it, otherwise I was not touching it.