Before walking up to a woman you always need to make eye contact and then see how she reacts to it, if she holds the gauge or better yet smiles at you then you should go up and talk to her. Although if she quickly looks away to avoid your gauge then your chances are slim, you should remember to scope the area and see which woman are looking for men and which aren't. Woman who are paying attention to there friends most likely aren't looking for a man to pick them up since they just want to have fun although on the other hand if a girl is looking around at men she's interested in getting picked up by one.
4: Don't express interest between her and her friends.
If you are paying attention and hitting on or showing your interested in not only her but her friends to you will come off as being a player to her and she will most likely avoid you. Remember to pay attention to her so it shows that she's your only interest not her friends
3: Don't avoid complementing her even if you think she has her them all before.
Woman may hear every complement about them but still love them remember an important part of getting a girl is to make her feel comfortable and appreciated.
2: Don't use pre-packaged pickup lines.
If you start a conversation with a woman using a pickup line you will come off in two ways either your trying to hard or your inexperienced with woman (even worse).
1: Know when to walk up.
No woman likes a clingy guy wait around and watch her body movements before you walk up, if she is bored with her conversation or looking at you, then go up and try to pick her up. Although if she is talking to her friends and having fun the wait.