What age are they and when is clean up time?
We clean up before breakfast and before afternoon snack. I've found that food is a great motivator.
I never say "Pick up or you can't eat" but it's "We'll have breakfast as soon as we're done picking up."
Cutting down on toys helps, there's less to pick up then.
I give them a choice: "Do you want to pick up the books or the blocks?" If they don't choose it's "3, 2, 1, you get books." If they offer to do something else instead that's fine.
Sometimes I just pick up toys and hand them to the children, with the "hard stare."
They usually just put them away.
I sing "Who, who, do I see, cleaning up the toys? I see _____, putting away the ______." (Row, row, row your boat) Pretty soon everyone is picking something up so I'll sing their name. The downside is that they stand and hold it until I do sing...
Every day children who clean up get a sticker. Some days the "best cleaner" holds the stickers for the others to pick. Some times it's random who holds.