Sounds counter-intuitive, but ride a fixie.
First of all, ride one with brakes.
Alright, now get it going fast enough to spin-out. You might need to go down a hill, unless is has a low gear. While you're spun-out, your butt will begin to bounce. Now focus on "pedaling circles." That is, don't just push down, use your toes to follow the pedals back, then up, the forward, and finally down again. You'll really be able to feel your calves.
When you're pedaling very efficiently, you won't be bouncing anymore: your body will be more like a well balanced machine. The bouncing comes from having your (heavy) legs moving out of sync with each other. By "pedaling circles", you are countering each leg's motion with the other and balancing them out: one leg goes forward, the other back. When you start pulling back and up in sync with the other leg (which is pushing forward and down), you'll be contracting your hamstring and calf and thus taking out any slack in your leg.
. . . so just keep practicing "circles"