I’ve seen different people mention in different places that when you look at girl in the eyes, you should look at just one eye. Anyone heard that? Can anyone explain the reasoning behind that?
I’ve never heard of that one but I suppose it could intensify the eye contact.
Years ago a friend of mine showed me a trick that works. when you get eye contact with the hot girl, look down to her lips (no lower!) then back up to her eyes, acknowledge her with a small nod or a wink then turn to her friends and start talking to them. If she’s interested, she will virtually jump up and down to get your attention again. Remember, the successful PUAs have the girls do the running to attract them, not the other way around. I’ve seen it work and had it work for me 
The ‘triangle’ that diamond mentioned is a great technique. I use it when appropriate.