This tip is one of the oldest around, but it’s so effective, it’s worth reminding you about.
Squeezing the tank/seat junction with your knees helps you stabilize the bike, and helps reduce the amount of upper body strength you need to use. That helps you ride harder for a longer period of time. It also works when you’re accelerating, or braking. (Just remember to keep your knees bent, and let them flex to help absorb bumps).
Here are a couple examples, courtesy of Chad Reed at Southwick.