I used luring with my boys. I put them in a down, and then rolled them onto their side (btw - I've trained my dogs to be used to me handling them this way, but for a lot of dogs it is VERY intimidating. Don't roll a dog over on its side if you haven't conditioned them to be used to it.) Then i got out a favorite treat, and slowly moved it towards the back of their head. Their head turned to get the treat, and their body followed. When they rolled over, I clicked (Used a voice marker for Ripley, who didn't like the clicker very much) and gave them the treat.
As they started to "get it" I added the command "Roll over" every time I lured them around. When they were rolling reliably with the lure, I just raised my hand up a little higher, and lured them from there. Now, it just takes a flick of the wrist, and they roll over.
One thing that I learned about teaching Rollover - do not associate it with the "lie down" command! I taught Ripley to roll over in several tries, but I unfortunately learned that soon, every time I told him to lie down, he anticipated the roll over command and rolled the instant he laid down! So, to avoid anticipation problems, when you put your dog in the down position, REWARD THE DOG FOR LYING DOWN, wait a moment, and then proceed to the "roll over" part of it. The two commands can get merged together and it takes a lot of work to "un-train" the dog from the bad habit of rolling over when it's told to lie down.