Wait till the come up to you, don't go chase after them. Then when they finally do approach you, and you guys are chatting it up, accidentally drop a 40 lb dumbbell on her foot. She will probably need to go to the hospital to have it put in a cast, offer to cut your workout short and drive her there. She'll see that you are really a great guy and fall for you on the spot.
It will work, trust me on this.