I actually imagine two streams - one positive and one negative - going in opposite directions. When a negative thought pops into my head (and it sometimes does just pop in there, which I suppose might support the theory that we can't control them), I take note of it and realize that I'm in the negative side of the stream (the water is murky and nasty on that side of the stream by the way). Then I visualize myself swimming to the other side and going in other direction (where the water is clear and cool). And I start thinking more positive thoughts. What I've found is that as I recognize the negative thoughts and make the decision to stop thinking about them, the less often they pop into my head. So, maybe while we can't control them, we can at least keep them from reappearing so often.
I will add that it is not an easy or quick process. I used to often get discouraged when the negative thoughts kept appearing. As I've kept on, however, I noticed a difference, which encouraged me to keep going.