give her your business card. has always worked for me.
Girls are usually not attracted to guys that hit on them. Immediately, it becomes "I could have him, he must not be anything special". I'm serious. Treat her like you would your brother. Tell her to hurry up on the machine, tell her that outfit is clashing, just be a major A-hole.
You'll become a mission for her. She'll start doing little things to annoy you, or impress you, etc. She'll be prompter on machines, or dress better. Then one day, she'll have a fight with whatever guy she's dating, and "banging the shit out of that A-hole in the gym" will be first revenge thing on her mind. She'll fvck you like you owe her money, a real fun ride. Then she'll be yours, and you can decide what to do next - play nice, and wait for her to bang another prick first time you fight - or get yours and move on.