1. Not wearing cotton. Silk and synthetic fabrics don't breathe like cotton does, which can create a breeding ground for bacteria.
2. Wearing the wrong size. Underwear that's too big looks saggy and messy. But too-tight underwear is even WORSE because it can cause chafing and irritation.
3. Wearing them to bed. It keeps that area from getting air . . . and there's the possibility of trapped moisture causing problems.
4. Putting them in the dryer. The heat in a dryer breaks down fabrics, so the elastic in your waistband will stretch out over time. It's better to air dry them.
5. Not buying new ones. A recent study found that you should be throwing out your underwear and buying new ones about every YEAR. It's because bacteria like E. coli can build-up even if you wash them regularly, and can cause some nasty infections.
(The List)