Never apply antiperspirant before heading out for a run. Think about the word: Anti combined with perspire means against perspiration. The purpose of antiperspirant is to keep you from sweating under your armpits, and that can be dangerous if you're running on a hot day. Your body needs to sweat in order to cool itself down, so you don't want to block even a small portion of your body from being able to do that. You'll smell bad — that's for sure — but you'll be much healthier.

If you're concerned about how you smell during and after your run (and, after you spend weeks or months training for your marathon, you probably won't be), purchase a product that calls itself "deodorant" only. Most underarm applications are both deodorants and antiperspirants, but you can find a few varieties that are only deodorants. This means that they don't stop you from sweating, but you'll smell nicer than sweat.