My 2 cents:
If u keep getting rejected, change something... i have a friend who does the same **** with girls over and over again, and gets dumped 2months later.... he dosnt learn.
I had some problems to, i went from "hey.. hihihihih, soooo what are u doing here, lovely weather" to simply "hey you, yeah you ... wait a min i want to talk to you".
Just find something that suits you best. For me is make eye contact until she breaks it, ignore her a bit, go in for the number, leave the premise.
My recommendation:
You shouldnt linger, dont give her the oportunity to start thinking(everything goes bad when a woman gets logical).
Keep an enthusiasmatic voice and bodylanguage.
Introduce yourself, hold the hand shake, tell her you have to go but want to get to know her sometime, ask for number, release hand shake, smile and wave as u gtfo.