here's a stretch we recommend to clients with plantar fascia tightness. if it's really uncomfortable at first, that's a sign you need to do it. Make it slow and gentle, start with 20 circles one way, 20 the other, and 20 point and flexes. Keep the foot as relaxed as possible, just using the hand and arm to move it.
http://www.youtube.com/watch?v=cBYJz6F_rC8
I'm also a big fan of RogueFLIP's suggestion above to use a firm ball of some kind to roll the foot over. I'll have clients do that with a golf ball going REALLY slowly, like taking a minute to go from just above the heel to just below the ball of the foot. When you find a really tight or tender spot, linger there for a few minutes and relax into it until it feels like it's melted away a bit.
In EVERY case I've seen of PF, gait mechanics have become compromised to the point that foot strike during walking or running is dysfunctional. The problem is almost always NOT in the foot, the foot is just paying the price for dysfunction elsewhere in the body.