Plantar Fasciitis orthotics
First, the arch needs to be strong and pronounched. fairly straight forward, if it's flat, don't get it. That typically means stay away from Dr Scholls(No Walmart crap for your feet!)
Second, make sure it's rigid but not too rigid. If you can easily bend it in half, it won't anything for you. but if it's made of stone, that won't help too much either. You need to find a balance between being rigid, but having enough flexibility that it doesn't over stretch your arch again(which caused the pain in the first place).
Finally, don't get too much cushion. a little bit is fine, but if your insole is 75% cushion, it isn't going to do anything for you. You need that orthotic to press against your arch just like you would press your thumb against it. if the arch collapses because it's made of cushion, it's not going to keep your feet supported.
If you want some great examples, check out http://orthoticsplantarfasciitis.com/. They are a site dedicated to offering insoles and orthotics that help with Plantar fasciitis.