I wouldn't recommend going after girls with boyfriends, but you'd be amazed what they will do and not think twice about it.
A few things that will help.
1. Be like a feather, fun and light. Don't get all serious on them, they have a boyfriend for that. Be a blast to be around, never take anything seriously. If you're not having fun, you're not going to get very far.
2. Never mention the boyfriend. That is her problem not yours. You're going to be tempted to say, "So, what's the deal with Bill" Don't freaking do it. Not your deal, it's all about her.
3. On that note, never ask about yourself. Always assume that she is interested, and she usually will be.
4. Give her the gift of missing you. You'll have a lot more success having her chase you, than you chasing her. She already has something that she can get anytime. The reason why she is attracted to you, is that you are a challenge. Don't forget that.
5. Realize that your mind will play tricks on you. You never like her as much as you think you do. You only like her because you can't have her. Keep that in mind at all times.
6. Have guidelines. Decide where you draw the line, married, separated, live-in boyfriend, casual boyfriend, engaged, etc.
Hope this helps.
Romeo