1. An amateur photographer is enjoying a parade when he discovers he has only one picture left on his roll of film (and of course has no more film.) He knows there are four more floats in the parade and he wants to maximize his chances of making his last picture be of the best float. What should he do?
2. What if there are 5 floats remaining?
3. 6 floats?
4. 10 floats?
5. 100 floats?
6. N floats?

[After a few responses, I'd like to add: There is not meant to be a trick to the picture taking. The picture may only include one float, and after a float has passed by, its picture may not be taken.]

Source: Extended from puzzles from Michael Winckler, Brendan Voge, and rec.puzzles.