Stand on something
I stand on something like a block or a stair that will get me up 4 inches or so off the ground.
It seems the best results come from getting as high on your toes as you can, and getting a decent stretch at the bottom.
I also like to keep the reps moving non-stop, rather than pausing in the stretch or at the top. I usually do 20+ reps, but seldom more than 30. It should REALLY burn during the last 4-5 reps. I stretch hard between sets.
You can also try putting a slight bend in your knees, just don't turn the exercise into a combination leg press/calf raise. This seems to work especially well on exercises that use both legs at once, like Donkey Calf Raises, or Toe Presses on the leg press, or any of the stending calf raise exercises.
Work to really FEEL the stress exactly where you want it.