Thanks to Karin for this Frugal Baby Tip!
This might sound a bit mean, but it's the only way I can diaper my Erick when he is h*ll bent on turning over while I'm trying to diaper him. I lay him on the floor an hold his arms out to the side and pin them down with my legs (my feet are up by his head) so that he can't turn over. This makes him pretty mad, but this is a kid that seriously hates laying on his back, and I just can't get a good fit if he's not laying down. He's been geting a little better lately, so maybe he's giving in to the inevitable...?
(Don't worry, it doesn't hurt at all, just gets 'em mad until you let them up again.)