First of all I don't want to sound like I came up with this info on my own. I've learned this from my favorite PUA's: Sterling and UnderScore. Check this out www.sterlingslounge.com for their blog.
IOI's are important. The more you take notice of them and capitalize off them the easier it is while you're out sarging.
My favorite one that I notice is proximity IOI's. When a girl stands or sits near you (especially if the place is empty). I notice this a lot a bars now. Even if she's just walking up and getting a drink.
I like to think of it the way UnderScore does. He says basically this is their way of approaching you without saying anything. He says to look at it as if they're approaching you and all you have to do is keep the interaction going by starting a conversation.
It's so much easier when you look at it this way. When I get these IOI's from HB's I will act on them right away now. I've found my success rate to be much higher when I'm approaching a girl who has already shown interest in me, rather than approaching a girl who hasn't given an IOI at all. www.sterlingslounge.com