OK. Well, your making progress. (sort of)
While standing and watching isn't the best way to get noticed, coming up from behind a girl isn't your best look either. I stress eye contact. It is important. Beside that, most girls don't like it. (as you can see from the responses) especially if you have a boner. How would you like it if a dude came up behind you and started rubbing his boner on you? (didn't think so) LOL
OK. remember, your doing better! one step at a time. Your not gonna get this all at once. Its trail and error.
I do stress making eye contact first. Then moving closer and asking if they want to dance.
Also, don't ever make the mistake of generalizing all women. I am sure there are some girls out there who will tell you that someone came up behind her and started dancing with her and she turned around and it was the man of her dreams and they fell in love and got married forever and ever happily after.
In general though, approaching anyone from behind for anything automatically puts them on the defensive. That's human nature. Yeah it happens all the time in the club, but you don't want to be one of those guys.
Just keep trying and keep asking questions.