Overpronation is a serious problem that afflicts many runners as this condition can slow down and inhibit your ability to run. One simple and inexpensive way to address problems of overpronation is to purchase orthotics or insoles which can help limit pronation and the other side effects that result from pronation. On the market I believe there are two orthotics or insoles that are ideal for the runner with an overpronation issue.
One is Superfeet orthotic which is highly recommended by many Podiatrists. This line of orthotics is one of the few that was actually created with a high impact runner in mind. It can be characterized to the type of foot you have based on the color of the orthotic. (however if you suffer from overpronation you most likely have a flat foot). The orthotic has a low profile and is very comfortable for ambulation and can absorb the stress. If you are looking for an orthotic for purely running this is the orthotic I recommend.
However, if you are looking for an orthotic to wear while running and performing daily activities, I recommend the Powerstep orthotic. The Powerstep has a deep heel cup associated with it to help reduce pronation. It will provide solid support and is more than capable of lasting through even the longest of runs. Consider the Powerstep as the swiss army knife of orthotics that can do and tolerate most anything.