I've become fairly decent at meeting girls at parties (in college) dancing with them. I'm a decent dancer and can play up kino to where I communicate confidence to the girls and we end up kiss closing on the dance floor.

Eventually I'll say something like damn its getting hot in here, wanna take a break outside. There we'll talk a bit and usually the girls ask where on campus I live. At this point we've only known each other for maybe 30 minutes. Unless they seem very receptive to go back with me I'll suggest going to get a drink or going back on the dance floor. In this transition back to the dance floor the girl will either run into her friends and make up some excuse to leave or well keep dancing repeat the whole process and then she'll leave.

I'd like to know how to keep from loosing the girl as there goes 45+ minutes of my night with someone who just ended up leaving me. Should I ask for a number when they say their leaving? What would be a good non-creepy line to do so? I feel that if they're just gonna leave they weren't interested to begin with and its not worth the effort. Is this just a shit test that I've failed or have I not gained enough comfort in the girl for her to feel comfortable going back with me?

Thanks for any input