Mpeg is great and if windows user, I have been playing with Huffyuv 2.1.1 at http://math.berkeley.edu/~benrg/huffyuv.html and have cut file size down by up to half and for working with flash have been playing with FLIXPRO at http://www.wildform.com. They have a demo version so you can check and see if it works for you.

Good luck as I got a client with same problems I know how frustrating it can be to webitize movies.