It's probably blindingly obvious to others but the following rules have served me well.
1. Never approach a group that is evenly distributed around a circle. So if 2 people are opposite or 4 or a square don't engage. Do multiple loops until you find a group you can approach.
2. Have a good magic trick. I have a very nice one that involves a pack of playing cards and then involves a lot of eye contact while I "read your face" to work out the card.
I have found 99% respond yes to "would you like to see a magic trick?". It works equally well on CEOs as it does students.