In addition to keeping your heels down, and shifting your weight to the back of your seat, pretend you are scraping mud when you reach the bottom of each stroke. This will empower your hamstrings and give your quads a little rest. This technique can be used all the time. You will become a stronger cyclist for it!

This piece of advice comes from various sources, the latest one was from www.roadbikerider.com. These guys have all kinds of tips and tricks for training. I've been reading their newsletters (once a week) for a couple of years now. Very useful. Signing up is free although you can also subscribe to the premium site which for a fee gives you more info.