I wear a sports bra (love the cute, strappy ones from SanDiegoFit.com) and a pair of short, stretchy running shorts. I can't stand to have anything on my legs or my midrift when I'm sweaty.
If I'm running outside, or it's a bit cooler, I wear a tank top over the sports bra. I've discovered a whole world of great tank tops by buying them in the little girls department at places like Target and Wal-Mart. I buy the size Large. They're usually about $3-$4 and fit perfectly!
I don't wear cute clothes because I need them to motivate me, or because anybody might see me. I wear cute/stylish workout clothes for ME! I love great clothes period, and workout clothes are no exception!