Don't coast down hills.
It's obvious to most racers, but many casual riders don't realize that pedaling down the hill will make a huge difference in your overall speed and make that next hill just a little bit shorter to climb. While you're going down that hill you might be thinking, "I'm too tired to keep pedaling, did you see that hill I just had to climb???" Just remember, pedaling down hill is easy. Nobody is saying you need to sprint down the hill, just keep your pedals moving, it will stop your legs from stiffening up and give you a bit of speed.