Kim,
What you are onto is a part of what's called "mindfulness meditation". An actual experiment was done asking people not to think of white elephants, while showing another group a white elephant but giving them no direction. Anyways, the group that was instructed not to think of a white elephant thought of it more often and not just in the hour or so of the experiment.

This knowledge is now use for people with OCD who have intrusive thoughts. You don't actually have to meditate, but like you said, just treat the thought as if you were thinking about "what you had for supper a few nights ago." It does work