Okay, this is mainly for guys.
Guys, when approaching a girl. First thing you want to ask yourself is "What do I want?" Do you want to make a friend, to start a relationship, or just talk to a hot chick? Okay, when it comes to girls, they have different reactions to everything.
If you're trying to start a relationship or a friendship, I wouldn't suggest using a one-liner. Most girls, will find it humorous but if you stay and talk to them, they'll think you're immature. So if you're a one-liner kind of guy or you want to say a one-liner to a girl. Tell it to her, she'll most likely laugh, then walk away.
If you actually want to start something big, It sounds corny, but be yourself. Girls can see through the us. They can tell if we're nervous or if we are totally fake. So when approaching a girl, take a deep breath and walk up to her. It also helps if you first make eye contact, so she will know that you exist before walking up to her randomly. Also make sure you walk up to her from the front, don't walk up to her from behind, that could be awkward.
Flirtation is key when talking to girls. You don't want to over do it, but you have to flirt a little bit just to let the girl know you're interested in them. Otherwise, they might just think you're bored and want to talk to someone. Girls don't always understand us and we never understand them, so trust me on this one I've gotten girl's opinions to back this up.
When talking to the girl, never ask her straight up if she has a boyfriend. Let her tell you. If you ask her straight up, then you'll get shut down straight away. Let her get to know you first and then ask her for her number, then if she doesn't reject you even if she does have a boyfriend, you at least made a new friend or if she doesn't give it to you because she does have a boyfriend, well now you know she has a boyfriend.
Asking for her number, this is probably the easiest part about talking to a girl. The hardest part is when to do it. Ask her at the end of the conversation. Don't ask her in the middle and definitely not in the beginning. When you finished having a long conversation, ask for it. Just ask, don't say anything cheesy or you might ruin the whole conversation.
After you have her number, start texting and calling her. Don't go by the three day rule. If you wait three days, the girl might forget who you are and then you'd get that awkward "Sorry, who is this?". No one wants that. The best thing I've noticed is after you leave and you have her number and make sure she has yours too, because that's always awkward too. So after you leave, text her and tell her is was great to meet her and you hope to see her again soon. Then the next day, call her.
Follow these guidelines and I promise you, you will achieve victory.