Chances are your mileage goes up during the warm weather months.

The combination of more miles plus sweaty feet can add up to blisters, a nuisance at best, but at worst seriously painful and a potential entry for infection. The key to preventing blisters is to keep your feet dry, certainly a losing battle in summer months.

But help may be found in your underarm deodorant as long as it contains an antiperspirant.
If you're prone to blisters, spread it on your feet routinely after your shower (effectiveness improves with regular use).

There are special over-the-counter foot antiperspirants available as well. Petroleum jelly on problem spots also helps, as do well-fitting shoes, and synthetic socks.