Great Post. If I may add my 2 cents:
I believe another great tool is Time-off (especially when you have known the girl for a long time). I.e. Cut all contact with the girl for a period of time (this should depend on how long you have known her...but the Max I would suggest is 4-6 months (if you have known the girl for ages).
Time is a great tool when used smartly. This "time off" would mean that you would leave the girl without the usual friendly banter/chat you shared and would wipe the slate clean. I.e. when you do get in touch with her again you can have a fresh start in a sense, and using the great tools in the post have a better chance at succeding at your game.
Another advantage of this time-off could be that the girl's situation might change, i.e. someone else could fill the "Friend's" place that you once held and she may not be with her BF anymore (if she had one in the first place).
Imp: You should try and come up with a good excuse about why you would be unavailable for a period of time...if its not possible to cut all contact, keepp contact to a bare minimum.