Don't you just love a challenge.. Well I consider the taken women an enticing prospect, you know a little bit harder too get!
My example:
Met a fantastic girl - no suprise she had a boyfriend. The saying the best ones are always taken is a very true statement, but no matter the best ones aren't always taken by the best guys..
Heres where I step in..

Met through a mutual friend so approval is already given.
Relationship talk:
I always love to highlight why being single is better, more freedom, I can do my own thing i'm just enjoying doing my own thing thing.
She has a semi needy boy friend.. Fantastic! They always do...
I highlight how i'm chilled out and we both have a laugh at how guys are becoming incredibly needy in this new day and age..
At this point she may shit test you saying you just like being single for the variety of girls, even hinting your a player..
Don't completely deny, girls love a rep

But rather brush it off and say something along the lines of nah you know theres a lot of beautiful girls out there but most girls really lack personality and aren't any fun to hang out with. Mystery's "Beauty is common" line pretty much the same.
Well you've probably heard all that stuff before but its necessary stuff anyway. So where were we.. After you know shes interested! No need to explain... (Isolate time) This is how you're gonna turn this nice taken girl to the dark side..
Take her for a walk.. I love walks.. Great form of isolation..
In isolation i like to imply that I don't wan't to bush her boundries considering she has a BF.. But do I really care? No. I could'nt care less..
So how do I like to do It? It works every time..
By this time i'm usually walking or sitting hand in hand.. (I know im in)
Me: So I probably wouldn't wan't to take it further than this tonight would I?
(Rejection Factor) - Haha who cares.. If you're good enough to get to
this point it's not going to happen!
Her: Ummmmm - She will usually umm an ahhh or make some noise of im not sure but i'm kinda keen.
Me: Cut her answer off.. I'm in complete control of this situation and I say: Wait.. Friend's kiss each other on the cheek don't they so it would be fine if I just gave you a kiss on the cheek..
Her: She says of course and she absolutely loves it!
Me: But wait now you probably need another one on the other cheek..
Her: She just takes it you don't even need to wait for answer - Make you're kiss longer and linger a bit afterwards.
Well she had an even better time with that one so now is the time too close!
Me: Hey even some friends kiss on the lips when they say good bye and thats perfectly normal.. How about we practice saying goodbye to each other?
Haha this may seem like a corny joke but if said right will have fantastic results..
So if taken girls are you're thing which im pretty much sure they are! Well of course they are! Try this..
She has a Boy friend who cares..
If you have any suggestions or alterations to this method feel perfectly free to shred it, abuse it, totally critique it or even use it..