This goes without saying, but I find a lot of guys get hooked on one girl and focus all their energy on her. If they get with her, that's fine, but if things don't work out it's easier to get hurt.
Case in point - I hang out with a group of friends once or twice a month and party. Recently this girl who went to high school with some of these people has been showing up. She's really cute and is in the same job field as I was in, so I was able to build comfort by knowing some of the same people she did and the business itself. The last time I saw her she went cold, though. She barely spoke to me and was hanging around a mutual guy friend who she's known for a while. A week later I see on Facebook that they're now officially in a relationship.
I dug this girl, but I knew to keep my options open because I was getting some weird vibes that things weren't exactly clicking. On top of her being with my friend, she's now damaged goods. I absolutely believe in bro's before ho's, so now she's completely off limits even if they break up. If this were me a few years ago I'd have put more effort into getting with her, then be devastated when she got with my friend.
So long story short - There are millions of single, eligible women out there. One girl may make you fall head over heels, but she's not worth any pain or headaches if you're not farking her. Keep your sights on at least 2-3 girls so you have a backup plan in case things don't go in your favor. And if you feel you're putting in too much effort, abort! If a girl is into you she'll be putting in the effort to be with YOU.