When I'm at a bar with a group of people and I don't feel like breaking away from them much I use this line. I also always either try to get a table thats either on the way to the women's bathroom (only catch them when they are coming out though to make sure they stay longer without distraction) or by the bar where most people are ordering drinks.

When she is walking by I make sure to make eye contact with her as far out as I can and give a sort of curious look. When she gets closer I make it obvious that I'm about to engage her so that she isn't surprised. She's usually curious as to why I'm looking at her with a curious expression. I'll stop her and this is the general interaction that follows...

Me: "Hi! Hold on for just one second"
Her: "Umm sure"
--- Look into her eyes curious at first and then slightly disappointed ---
Me: "Ok nevermind. Thanks anyways though"
--- At this point make it to where she can still engage you but you should be slightly disengaged and kind of - moving away from her showing a loss of interest---
---She'll be confused and curious as to what the hell just happened. In her mind you showed interest, made contact with her, and then broke away with no explanation.---
Her: She'll laugh to kind of get your attention and to break that moment that was awkward for her "What was that about".
Me: -Being friendly but not overly interested in her "Oh I love girls with "X" color eyes but you don't have "X" color eyes. Yours are "X".
--- She'll be thrown off by this. Normally guys are after her for her body and something like her eye color is never an issue. You've established you have standards and that she isn't meeting them. Begin her qualifying herself.
---This is generally where two paths take place. I'll describe the most common one first---
Her: "Really? You aren't going to talk to me because of the color of my eyes?" (Something to this effect)
Me: --Acting kind of shocked like I really hadn't thought about it like that.. "You know what I guess that is a bit shallow of me. Tell me something interesting about yourself then?"
--- Now she'll really dive into trying to qualify herself to you---

The other path is she walks off offended to tell her friends about how rude you are. This isn't a bad path for the line to take either because now all of her friends are also looking over at you wondering why you just shot a beautiful woman down over something as trivial as the color of her eyes. They'll either confront you if theres a "alpha female" that has to defend her group or you can open up the girls individually on their own. Opening them when they split off from the group works great because you can engage them and seem interested and they constantly be thinking "OMG he blew her off but he decided to talk to me" makes her feel like she might be better than the rest of the group!

If you do get the alpha female attacking you have fun with it! My favorite first reaction is when she starts complaining ill say "Whoa whoa whoa slow down there tinker bell" (for some reason women love being called tinker bell, its weird) When she pauses look at her eyes like you are making the same judgement. It'll piss her off. Then before she can start up again give a cocky type smile put your hand out and introduce yourself in a really friendly way. This completely crosses her wires and will fry her brain. It's all game after that.

So give it a shot let me know how it works for you or if you have any advice that you think might raise the 70% up a bit then throw it into the mix!