Here's the stretch I was prescribed by my physical therapists that has helped me a LOT.
Get a square board, and prop it up at an angle--maybe about 45 degrees. Put your right foot on the board, with your heel on the floor. Keeping your right knee straight, lean forward until you feel a stretch. Hold the stretch for 30 seconds, release and repeat. You really need to hold the stretch for 30 seconds, and repeat it 3 times. Most of us don't hold our stretches nearly long enough to do any good. Then, do the left leg.
The next stretch starts off the same, but this time, when you lean forward, bend your knee of the foot that is on the board. You should feel even more stretch a little lower, from your achilles and down along the bottom of your foot. Hold each time for 30 seconds.
My plie has gotten a lot better since I've been doing these stretches. The first time I held these stretches for 30 seconds, I got sore, and my therapist said to reduce it to 15 seconds and work my way up. So you might have to do that at first.
By the way, I actually saw a dance magazine selling a wedge shaped thing that you would use for this exact purpose. But really, you can just prop a board up at an angle for free.