Most of the time they are looking when you're not looking. It takes a lot of confidence to look a stranger of the opposite sex dead in the eye. Since it pretty much leaves you exposed. Obviously you are interested, and many people view this interest as a powerless position. That however is a common misconception.
Furthermore is a ton of common tactics women use to get your attention, if you aren't awae of them then the odds that you miss the clue are pretty high. There is also that factor that if you don't have the right kind of attitude, the right kind of confident manner, you are much more likely to miss the clues, since you are to eager to dismiss them in your mind.
Simple example... I've often noticed that it's a seemingly popular tactic to pass by and all accidental like 'drop' something right in front of you. I noticed this quite a few times. I once busted a girl on it, she denied it... then two later i asked again and she confessed that was a scheme to get my attention... interesting really
There is also the matter that for a confident person, it's very easy to tell if another person isn't confident with him/herself. It's kind of like the seeing observing the blind more or less. you may not notice it of yourself but you are sending nervous/anxious vibes.
The way to draw eye contact is actually very simple. It's more difficult in crowded places of course, after all, why would someone from across he room be making an effort to look in your direction, unless you've made a conscious effort to draw attention. However, it's a basic human characteristic to want to be aware of their surroundings, and that means observing other people in their surrounding. The best way to make eye contact is to look in your own surrounding... so if it's crowded place you'e best off taking a tour of the building. You may not be aware of this, but people are almost always aware of your presence, when you walk by, they noticed you. However this process is so subtle that even the people noticing you may not be fully aware that they did. But you can change that if you wish. If you're walking by an attractive woman, just let your eyes do the talking, look her dead in the eye and when she looks back, keep looking. Some will dodge away, other will smile and say hello, either way they got an impression of you, they are aware of you, and it's a good way to invie yourself for a conversation.
It works on guys two, although i don't really recommend it. Some guys tend to view eye contact as a challenge and they may feel the need to step up to the plate
. Even so, most of the men i've pulled this on (i like to experiment) have simply given me a friendly nod... One guy not so long ago even laughed after staring back hard for a while, and came over and said something along the lines of "nice, you got balls, i don't notice that in guys a lot"... Was kinda funny really.
I'd recommend that you keep your eyes level, don't be temped to wander to more 'interesting' areas, unless you want to lay the sexual inuendo on inches thick from the get go. If so you might aswell take your times doing it. It isn't always received well though. Some women think it's funny in an outragious sense, others think it's attractive, and others just think it's offensive. So yeah, use at your own peril.