The downward facing dog is a stretch taken from yoga that I've found really helpful for stretching tight muscles in the backs of the legs and calves. Here are the basic instructions for the exercise:
How to perform the Downward Facing Dog
1. Start on your hands and knees. You hands should be underneath your shoulders and your knees underneath your hips
2. Lift your hips up so you body forms an inverted “V” shape (as shown in the picture)
3. Look at your feet and try to press your heels down toward the floor
4. You should feel a stretch through the back of your legs and into your calves. Once you feel the stretch hold the position for 30 seconds to 1 minute and repeat 4-5 times
Exercise Tips For Flat Feet
When I first starting stretching with the downward dog I wasn't able to get my heels flat on the ground. After working on the exercise for a few weeks I was having more success getting my heels down. The problem though is that my arch was collapsing in that process. I was pronating my foot to compensate for the lack of flexibility in my legs.
To correct this problem what I began focusing on not letting my arch collapse, even though it meant my heels wouldn't make it down to the ground. I did this by actively inverting (turning in) my heel. This motion is hard to describe so I've included a picture of what inversion looks like. Essentially, you are trying to turn your heel in so that it points toward the opposite heel.
For the downward dog I would get into the position where I felt a stretch, and then would hold the stretch while keeping my heels inverted. I did this exercise a few times a week for several months and eventually I was able to get my heels down while still keeping an arch in my foot.