walk in and yell out, TOPANGA!
All kidding aside, just go up and talk to her. Don't be scared. Maybe get a number out of it? As for generally hitting on someone though, it's not really my thing at the gym. I work out fairly often, but I'm there more for myself and not looking for girls. If a cute chick walks up to me, that's fine, but I won't go approaching girls at the gym.
As you said, you're tired, your sweaty, you're not looking your best at the gym (aside from the muscles lol). Work on yourself at the gym for your own self-improvement, work on the ladies everywhere else.