How can I stop coming across as the nice guy?

A girl rejected me because I'm too "super nice". But everyone says that, and it's time I stop being it. Even I find it try and I want change now and am willing to put forth effort and time.

Ok, I know the basics, stop agreeing to everything, don't let people step all over you, be bold, be assertive, but how can I actually do this? Where can I practice this? I keep hearing to just try them out of nowhere but I'm sure it just doesn't happen like that. Any good books as well to read or self-hypnosis tapes? Also, I'm supposed to pick up my sweater from her on Sunday. Anything I could say or do to make it seem like I'm not that nice and make her be like maybe she was wrong? Thanks Yahoo Answer community, any help is greatly appreciated.