"The One Line That Makes Women Curious"

This is one opening line that has gotten a response rate of 100%:

"Congratulations!"

A woman's curious nature will force her to take the bait and respond with "For what?"

You then say anything remotely clever and you're in. Personally, I like to rip on them such as, "That's the goofiest outfit I've seen in the gym today." Or "You've been spending too much time working those calves. You make more noise walking than anyone I know."

Say it playfully and with mock horror and you'll be into a conversation that starts with her laughing.