I try to think of the most disgusting thing I can when if I feel like I'm gonna cum. Sometimes it works, sometimes it doesn't.
Try thinking of dead kittens, not just dead, but like smashed in the street with guts everywhere kitten.
Or a really fat man you know. Don't think of him naked or else your mind will stay on thinking about sex. Just think of him with like a full slab of ribs or something.
Or someone who's really old and decrepit.
I can't really think of any other ones I use.