Make sure you're not too far forward over the handlebars. You want your hips to be about 10-12" in front of the seat post for balance. Pull up on the handlebar on the side that you're pushing your leg down on. (The other arm is just relaxed.)
You can find out why pulling up on the handlebar works by putting a bathroom scale in front of a dresser. First, lean forward and press one leg down hard on the scale. Make a note of how many pounds register. Then, slightly pull out one of the drawers just above knee height and pull up on it from the bottom while pressing the same-side leg down on the scale. You'll notice a huge increase in how much pressure you can apply.
And, of course, don't forget to use your hamstring muscles to pull the pedal around and over the back side of the pedal stroke. Think of your leg circling around the hip joint like the Road Runner in those old cartoons.