"Accidentally" bumping into them with my boobs, then feigning embarrassed apologies while making eye contact usually got a conversation started. 42H's were handy that way. A big booty can get the job done too.
The art of the approach is to leave the dude thinking it was his idea to get things started.
If you're not comfortable with direct physical contact, then eye contact is always good. If you're new at it the best deal is the 'let him catch you looking then look away shyly'. If he catches you twice and doesn't come over then move on to the next one. Practice a signature 'come and get it' looks in the mirror. Lots of women think they have these down but they just look constipated.