Dating a girl who has a boyfriend requires a lot of tact, intelligence and patience. This is not just the normal boy-meets-a-girl kind of a scenario. Here are tips to help you get a girl who has a boyfriend:
Remember that she has met very many average guys who tell her what they want immediately. Saying what you feel for her will scare her away and put a wall between you. If you approach her like any other guy she has met before, she is likely to reject you.
Try to befriend her
Respect her relationship with her current boyfriend. Do not try to attract her as that will translate into cheating, and chances are high that she likes her boyfriend will not cheat. Since you already know that she is unavailable, start by trying to befriend her.
Get into her world
Once you have made it clear that you are not the average guy who wants to lay her, you can now take the next step of entering into her circles and her world. Invite her to a party and even ask her to bring her friends. You can even go the extra mile and invite her boyfriend. This will completely disarm her and open more opportunities for you to date her friends.
Wait for the perfect time
The more face time she has with you, the the easier it is to date her. You already have access to her social circles and her world. Continue hanging out with her and her friends and in case one day she falls out with her boyfriend, you will be the next available date.