Only farmers ride with their calves
My first dressage instructor drilled that idea into me.
Think about riding the horse more from your core (thighs and seat) than lower leg. Everyone says this but it really does help. If the aids come more from the thighs, the lower leg can be passive most of the time.
Hip flexors are key. Here is a good series of stretches http://www.youtube.com/watch?v=jg3y96ijATw
Key points for safety and comfort;
Optimize the separation of your legs-too much and you risk a groin injury, too little and it is hard to maintain balance. I kneel and have about 6-7 inches between my knee caps.
Make sure your foot has enough separation from your body that as you come forward, you knee doesn't go past your big toes. Also keep your shoulders over your hips, don't let your upper body fall forward toward your thigh.
If you do warrior I, think of driving your back leg down while curling your pelvis forward a little. That stretches out your hip flexor and top of the quad pretty well too.