Getting out/eating less fast food and soda/excercising more - those things will all have a massive impact. And the 'getting out' thing is key. Even on a day when you can't excercise formally, just getting out and walking around briskly keeps the metabolism up.

For me, I like to excercise at the beginning of the day - by the end of the day, so many things can come up to derail my plans... (Plus, it wakes me up and makes me feel more energized for the day.)