Plantar Faciitis is basically tendonitis of the tendon in your arch. The solution ultimately is to stretch and loosen it back up. Variations of the Strassburg sock, such as Jimincalif linked to, work. You can also freeze a pop bottle full of water and gently roll your arch on it a few times a day to help stretch things. The cold prevents swelling. You could also try to apply heat prior to the ice-rolling, to warm up the tendon first.