S'wheat scoop (wheat clumping litter) has a great affinity for bonding to the litterbox, and it's like cement when it does (you could build a parking garage out of the stuff, I swear). Spraying the box with PAM usually helps immensely...might work with clay clumping litter, as well. My co-op sells S'wheat, so I have passed this tip on to many very frustrated organic food weenies, and they've all said it works well.
World's Best Cat Litter (made of corn) doesn't seem to stick to the box at all, but it does tend to track pretty badly. I have this thing about clumping clay litter, and just can't bring myself to use it, but I haven't had any cement issues with WBCL.