You should lubricate the belt of your treadmill every three months (dependant on the amount of use it sees). To do this, check with your manufacturers manual and follow the instructions on how to relax the belt on your model. Once this is relaxed, you can lift the belt strip, and apply a light spray of WD-40® High Performance Silicone Lubricant to the underside surface.
Next, re-tighten the belt, and allow the treadmill to run on a low speed, so the product is distributed throughout the whole surface. Ensure you do not over or under tighten the belt, as this can lead to dangerous accidents. Once this is done, wipe away any excess product that may be apparent, and double check the belt is secure before use.