What is the best way to show I am sorry?

Somehow I doubt anybody really knows what you did to your friend other then yourself. I'm sure you can work at being a better friend. (Even though you already are) Your boyish charm should take care of the bad deed you did. Make sure you are exra nice in the next few weeks and the problem will resolve itself. You should really make sure you return there phone calls, emails and texts as well! You don't want them to think you are sore about the whole situation!:>)

It seems you already apologized a few times and it's understandable that after the verbal apology the "feelings" of hurt still linger. As cliche as it sounds, only time will soften what has happened and heal the emotional wounds. In the meantime, what you can do is treat the relationship gently until that sense of normalcy you once had reveals itself again.

And then, when you feel upset or angry again, take a few minutes to breathe and contemplate your response instead of acting on impulse.

show up at her window and serenade with an apology song do something that will make her giggle and apologize. so that will give her a great feeling and for you as well to see her happy again.

Go outside her window with a boom box playing the song

Sorry - Buckcherry

It's nothing about love and it had gotten me out of lots of situations

just say " I promise I won't try 'that' again with you, trust me"

No need to mention what the actual deed was, although we all know what you are referring to...