In my past experience, it's a little bit tricky to work with girls with a boyfriend. I mean, how would you like it if you were her boyfriend, and some punk tried putting the moves on your lady?
Anyway, my mentor once gave me this rule of thumb. Now class, repeat after me:
'No means SLOW'
Tension is built when you give a little, then pull back, give a little, then pull back, and when she gives you a knee-jerk reaction, such as this, that's when you pull back.
A girl with a boyfriend is just like ANY OTHER GIRL. You know you're succeeding when two things are happening:
1) She shows interest
2) She shows compliance.
If she's showing enough interest, then you gotta come up with a way to lead her. If she follows, then you know she's being compliant.
Start with a small lead. Sit her down somewhere, stand her up when you're done, twirl her on the dance floor. Anything small. If shes jumping through your small hoops, then you're on good shape. Give her a bigger hoop to jump through, like asking for a favor, inviting her someplace, stuff like that.
Just BE PATIENT...let the tension build over time. Thats exactly what I did with a girl with a boyfriend....and as time went on, I foreshadowed a time we could just let out all our sexual inhibitions.
By the time she was naked on my couch, she was already onto the NEXT boyfriend...but she knew she wanted me. Bam, it was done.