I am trying to get my 10 year old to help me prep the laundry room for painting and learned that he will do any of the high things as long as it means he gets to get on top of the upright freezer, washer and dryer. He even hurried to wipe down the baseboards so he could scrape the area above the w/d. (Painting any room in this house is a major chore because I HATE the texture that the builder put on the walls and each room gets scrapped down, then lightly sanded before the painting can start.)
Then Jeffrey (12) came in and he agreed to clean the front of the cabinets in exchange for a turn on top of the freezer. There is an air conditioning vent that blows right on you while up there and with the dryer going the room is not very cool; I wish I could get up there and work.
In a different life (while mothering my 28 year old) I could motivate her to clean her room so she wouldn't have to eat a banana split!
Another time we spun a long drawn out fairytale about the toy snatcher that would come into your home and steal toys that were not put away. ;( They wondered right away why the snatcher had not been there before. (smart kids) I told them that they had obviously been kept a secret but I was willing, and did, make a sign that said, 'toy snatcher; CHILDREN live here' and went to post it in the front yard. The toys were put away before the poster was finished. I kept it behind the couch for awhile and just threatened to bring it out occasionally. (I probably damaged my kids for life-- 'cept they seem to be all right.)
The best ever was to convince a young child (about 2-3y) to allow his mother and 2 teenage sisters to kiss him. We told him that he would turn purple if he did not get enough kisses; he continued to allow a ridiculous amount of kisses. One night after he was asleep we dotted his body with a purple marker, alas he was not convinced.