I've taken xanax (alprazolam) on and off for a while. maybe one .5mg one day, then none for a week. as needed basis. how does this affect dependence/withdrawal?

You won't get either: Dependence on benzodiazepines requires daily or near daily use of enough of a dose to start re-regulating our receptor systems. Occasional use, such as you are doing, will not do that. However, consider using other techniques to control anxiety - meditation, self-hypnosis, progressive relaxation exercises, guided imagery and many other techniques are better than using benzos, even occasionally.