1. Men hit on you all the time, and you get dates, but you struggle to get (or keep) a boyfriend.
You are hot but your personality needs work. Men are attracted by your initial shine, but they soon discover your lack of substance. They don't like spending time with you because you are either boring, too high-maintenance, stupid, too insecure or annoying - or some combination of these. Be honest with yourself in determining which of these are the biggest problems, then re-focus your efforts and do whatever it takes to fix them.
2. You never get hit on by men you are attracted to, or else they hit on you but then make an excuse to leave and don't ask for your number. They ones that want you are always short, or fat, or ugly, or awkward. The hot guys that you want to hit on you never do. Or, you never get hit on at all.
This happens either because you aren't making yourself attractive enough or because you have unrealistic expectations about what kind of men you can attract. Assume the former first: lose weight, spend more time on your hair, or work on your appearance in some other way. Then, if things still aren't working out (and be patient because this takes time), lower your standards.
3. Your friends always get hit on and you don't, or you always get hit on by the less-attractive friend or the obvious "wing man." It happens so much that you just expect it now.
Same as #2, with the added suggestion that - if none of that works (and again, it takes time) - you should start going out with new people who are closer to your level of attractiveness, at least occasionally.
4. Men hit on you and ask (text) you to go out drinking, but not to go out on dates. Or they ask you on dates, try to sleep with you a couple times, then never call again.
You are projecting a slutty image by your behavior. Men immediately recognize that you are not girlfriend material so they just try to hook up with you. Tone down the drinking, leave bars earlier, and acknowledge that your lifestyle is not conducive to finding a guy who will respect you.
5. Men treat you like a friend, not like a girl they are interested in sexually or romantically.
Most of the time this is because you aren't very attractive but you have a great personality, and you aren't bad looking either (if you were, men wouldn't even be friends with you). You also probably allow men to treat you like one of the guys. This isn't actually that bad of a situation to be in, because with just a little effort, you can improve your looks to a point that men will look at you in a romantic and sexual light. And if you start acting more feminine you can break out of the "friends" mold.
6. Men hit on you and then text you, but never call.
This is the unfortunate way of the future. Get used to it. But, give a lot of credit to the few guys who do pick up the phone and call: they are either more interested in you than the others, or just more confident. Either way, they are a step ahead of the rest.
7. You know you are attractive and men always check you out, but you only get approached by cocky players.
This is a difficult situation. It means you are extremely hot, and you intimidate most men. However, your personality is more to blame than your appearance. There are plenty of hot girls that get hit on far more frequently than you. Adjust your look first, because this will be the easiest part: dress a little more cute and a little less sexy. More importantly: loosen up. The women that are most intimidating to men are the ones that are insecure themselves, and mask their insecurity by closing themselves to social interaction under a facade of "hard to get." I've met a few girls like this - drop-dead gorgeous but perpetually single. They are all the same in a nightclub or bar environment: they stand alone or with their friends, and don't have fun, or are too concerned with looking "cool" or like a celebrity to relax and enjoy themselves.
Let me know if I missed any categories, but I think that covers most scenarios.