The way I did it, I never really asked, but I also didn't go to part time. I just started spending more and more of my time helping people learn how to do the job better, and then building tools to help when it seemed like no amount of training would help them get better with the tools we had.

Over time the boss saw the value and made that my job without me asking. Of course, that requires a decent boss, and it also happened slowly over several years. I'm pretty sure he would have let me go part time if I asked, since he saw me as more productive in 1 hour than most people are in a week. At one point he told me to go to Amsterdam and work from coffee shops for a few months. I didn't do it, but in hindsight, I kind of wish I had.

I have a different boss now and things are drastically different. A boss can make a world of difference.