My ex-wife is a marathon runner. On the rare occasions when she got a blister [usually from getting her feet wet] she first washed the area really well with something anti-bacterial, drained it, and put on a piece of Spenco or moleskin MUCH larger than the blister and much larger than you would think necessary. The object of the game being to spread the pressure, and make any friction more difuse. Leave the dressing on as long as possible.

Worked for her - I have never seen her incapacitated.