1. say something strong and passionate along the lines of "I've been wanting to do that since I first met you", then carry her off to the bedroom.
2. act totally shocked and confused about what happened, like it was spontaneous - "Oh... I'm so sorry... I... I didn't plan for that to happen".

Although alternative 1 has the advantage of being very flattering forthe girl, alternative 2 usually works better. This is because:

• a) alternative 1 can be a bit too strong. No one likes to feel they've been stalked or hunted.
• b) if the kiss made her angry or upset she doesn't react as violently. It doesn't seem as if you're to blame, just that you were so overcome with desire that at the spur of the moment you couldn't help yourself, and would never have done it if you were thinking rationally.
• c) the girl is totally flattered that she is attractive enough to cause irrational behaviour, like an spontaneous kiss, in a guy.
• d) if you get this bit right she's yours - a surprised reaction may fool her into believing that the kiss was at her instigation, or at least that she is as much to blame as you.
• e) it makes you seem sensitive.

Lastly, if she says "I had no idea you felt that way about me", a clever response is "I think I only just realised it myself". Act full of wonderment and inspired awe at the realisation. This transforms you from a sneaky guy who's trying to worm his way between her and her man into the perfect man she's destined to be with, if it wasn't for that slack boyfriend of hers. It's no longer you that's forcing the two of you together, but fate.

This help is from a girl, incidently. I'm a hardcore serial boyfriendstealer, but frankly I find guys too easy sometimes. Not enough challenge. Hope this advice works for you, you see once I got stolen bya guy who used alternative 2 on me. I only later found out he'd been planning it for weeks.

GS:
This sounds like it just might work. Thanks for the thoughtful analysis. Let's get some more details about your boyfriend stealing exploits.