I developed this problem in my right foot at the end of last school year -- I had no idea it was so common. In my case, the doctor blames it on two things:
1) I'm a teacher, so I stand all day; some days I literally stand all day except for my 26 minute lunch break. I also have to park my car 1/4 a mile from my classroom. I just can't sit while I teach -- I'm way too animated -- but when school starts again I'm going to make more of an effort to sit during my planning period.
2) I started running with my daughter, who wants to go out for track next year, and I didn't bother with running shoes. Apparently Keds aren't good for running. In fact, I think the doctor's head spun around in circles when I told her what I was using.
I was confused about the pain: I didn't understand why it would be painful first thing in the morning, but less painful after I'd walked a while. The doctor explained to me that the muscle hurts when it contracts, but once it "limbers up" a bit, the pain goes away.
So, she assigned me some exercises: the golf ball on the floor thing has been doing me some good, but a frozen juice is better -- she said that the cold helps, and I"m supposed to keep rolling it as long as I can stand the cold. I keep one certain frozen juice in the freezer now, which is for my foot; we don't plan to drink that one! I usually do this while I'm sitting at the computer. She also gave me some stretching exercises and told me to buy better running shoes.
I've been following her instructions all summer, and my foot rarely hurts anymore.