We have a cock shaped dildo that we use on occasions and one time I mentioned to my wife that I would love to catch her using it on herself sometime unexpectedly. One day I came out of the bathroom and there she was lying on the bed using the dildo. Naturally I was immediately aroused.

I love the thought of her playing with herself when I am not there though I don't think she ever does it.

Put some porn on and tell her you would love to watch her pleasure herself while she watched the movie and you watched her.