I can't remember where I originally heard this strategy, but basically you walk around anywhere in public where there are girls and scan for eye contact. If a girl is walking your direction, stare at her but don't smile. If she returns eye contact and smiles, then you smile back and open her. Supposedly this weeds out girls who aren't single or aren't looking, thus making your process more efficient as you aren't just opening every girl you see and wasting time.