The "normal" lower limb has a slight tibial varus angle or in other words is curved slightly inwards. As a result at heel-strike the outside surface of the calcaneus strikes the ground which is followed by eversion (lateral/outside shift) of the calcaneus and pronation (flattening) of the arch of the foot. This along with other biomechanical events occurring in the foot and leg allow for shock absorption at the "contact phase" and begin the repetitive cycle of shock absorption and propulsion in the foot that occur with walking.

Overpronation (which can result in a number of different painful conditions) that occurs due to an excessive varus angle of the tibia and/or calcaneus can be corrected with posted orthotics which can be placed inside a regular shoe. Wikipedia reference-linkPlantar Fasciitis is reduced by preventing overpronation and midtarsal dorsiflexion and thus reducing the traction force on the plantar fascia.

I've fabricated custom fit orthotics using orthotic blanks and wedge strips for posts. I usually charge $25 for the materials so they're not really expensive but they're not as durable as you might expect to get from a podiatrist for example.

As has been mentioned above other biomechanical factors at the ankle, knee, hip, pelvis and spine need to be considered. For example, an anterior innominate rotation in the right pelvis would result in an apparent leg length deformity (right lower extremity longer than the left) and the right foot would tend to overpronate in order to "shorten" the right leg as a compensatory factor.

Finally I've recently come across an orthotic system designed by a gentleman named Phillip Vasyli. He is an Australian Podiatrist and he's worked in collaboration with people like Jenny McConnell (for the non-physios she's a world renowned phsyiotherapist) and has some really interesting theories. For those of you looking for shoes and/or orthotics he has some products that may work for you (I haven't tried or dispensed these myself so I'm in no way endorsing them).