Please please please stay nice! The world has enough a**holes, trust me!
I will admit that there are girls that love the "challenge" of a jerk, or maybe think that they can "help" him somehow - they will be the ONE who can truly see in his heart and help him be the wonderful sweetie she just knows that he really wants to be. WHATEVER!
Most girls get smart and tire of the jerks. You just have to catch one at that point. It's tricky, because usually what happens is this...
1. Girl dates Jerk
2. Girl breaks up with Jerk
3. Girl dates Jerk #2
4. Girl breaks up with Jerk #2
5. Etc. etc. etc. for awhile
6. Girl realizes Jerks are no fun
7. Girl starts looking for Nice Guy
8. Girl meets Nice Guy (or, more commonly, starts dating Nice Guy that was there all along)
9. Girl ends up in Long Term Relationship with Nice Guy
10. Usually, Girl marries Nice Guy
You know it's true - how often have you seen people you know never really seem to find someone that suits them, then all of a sudden they meet someone new and BANG! they get married.
You gotta catch them at Step #7! That's your window of opportunity. Have faith, my friend, you can do it!
One final warning though...
The main reason that nice guys get a bad rap is that many of them are WIMPS. It gets really annoying if a guy is like, "whatever you want to do is great with me sweetie" all the freakin' time. Especially when you know that it's not fine with him - he's just saying so because he's so desperate to keep you happy.
So be nice, be sweet, give sincere compliments, heck, buy flowers. Just make sure that you always keep your own personality and opinions. If you try to lose your personality to keep her, you'll lose yourself in the process, and eventually you'll lose her too. The magic of a relationship is getting to know the inner essence of another person and if they just agree with you all the time, then you never get to know them.
Good luck!