Wombat's post reminds me of an important element in making humour games work: most of the laughs should come from the PCs. Don't overwrite as the GM. Present them with the wacky situation, have a few story points and spot scenes to work in when there is a chance, and just let the players run riot thereafter. This works a lot better than trying to run a traditional narrative.
Oh, and keep the sessions shortish, or have lots of breaks. People need to relax and wind down, or the humour gets forced and stops being funny.