I have read recently, more information on preventing plantar fasciitis with foot strengthening exercises involving rolls in foam rollers, lacrosse balls, etc. As I sit here writing, I have a lax ball under each foot and I am rolling it out. I have never had plantar fasciitis, but I know several people that have and it sounds like it sucks.
Grab a ball and roll it out. Tennis balls can be used, but I have read that they aren't as strong. I would imagine that if you had one of those super bouncy balls from when you were a kid (or steal one from your own kids), that would work well too.