Plantar Fasciitis is a very common and painful foot condition experienced by many hikers. I know, I've had it. It is a repetition injury often caused by hiking or running and presents itself as chronic heel pain. This pain is caused by the inflammation of the ligament that connects your heel bone to the ball of your foot.
I've found that the best way to cure plantar fasciitis is to simply stop hiking for an extended period of time, like over the winter, AND to get a pair of boots with better arch and heel support. While it is possible to reduce the pain temporarily through stretching, applying ice packs, and taking anti-inflammatory medications, you need to give your body time to heal and takes steps to prevent a recurrence.
Step 1: You should stop hiking, possibly for a few months. Check with your doctor if you have any questions, but rest is best.
Step 2: Find someone at an outdoor retailer who really knows how to fit a pair of hiking boots. REI provides select members of their staff with this training, so you might want to try there. Call first. These people are worth a drive. Next, get some new boots that cup your heel very snuggly and replace the factory foot bed with a after-market one from Superfeet. Most factory foot beds that come with boots are a complete joke. Superfeet has different models with different degrees of firmness and arch support so you will need to experiment. It's also possible that you may need custom orthotics, particularly if your arch has fallen. Next, go hike about 50 miles to break in your boots and the foot beds.
Step 3: If you can, lose some weight: either body weight or pack weight. It will help reduce the stress on your feet.
The following online retailers sell Superfeet Insoles: