If you don't live in a climate where you need a pair of expensive waterproof boots or you just don't want to buy any, Instructables user csymmank has a guide to making slipcovers for any shoe that will work in a pinch.
Using a pair of socks, velcro and duct tape, the process doesn't take a lot of effort, but the fact you can easily remove the covers makes it far more appealing than just wrapping your shoes in duct tape. While csymmank's guide is all about using these as bike shoe covers, it's easily applicable to any shoe. The slipcovers aren't pretty, but they'll certainly come in handy for that once-a-year storm where everything get drenched. Find the full details over on Instructables.