Hellobee Boards

I technically went from 100% to 80%. I have to log at least 32 hours/week, but if I work more hours, I get paid the usual hourly rate, but all my benefits are pro-rated to 80%. I love it- generally it means I don't go in at all on Fridays, but it also means that if I need to take off a couple hours on a Tuesday for an appointment, or go home early because I'm not feeling well on Monday, I can make up time on Friday (very useful during pregnancy!)
It's common at my workplace- TONS of people have some kind of alternative work arrangement, and no one is surprised to hear that someone with small kid(s) works less than 80%. 80% is pretty common- not many people do less, but my boss was technically 60% for a while though she ended up working a lot more hours pretty often.
So I'm probably not much help, since there were already tons of precedents at my employer. My employer does require that you fill out a plan with your supervisor of how to make sure the change in your schedule doesn't have a major negative impact on your projects- maybe that's something you could do in advance of bringing it up to your employer. I also feel like having Fridays off helps me be more productive on other days, plus it means that I can schedule almost all of my other errands and appointments for Fridays and I'm rarely out of the office for such things on other days.

I did...sort of. During my maternity leave, I negotiated a part time job share, (which they gave me) but ultimately I ended up leaving all together due to my husband getting a new job and having to move somewhere where it was not feasible for me to commute.

I basically presented a plan to my boss. I found another coworker who wanted to job share as well. (They wouldn't let us go part time or work from home, the only option was to job share, which means the two of us is basically one person). What we negotiated was:
1) 50% salary
2) No benefits since we were working less than 30 hrs/wk (and in CA you need to work 30+ to get automatically get benefits) But this was okay because I could get on DH's benefits.
3) We would take turns working 2-3 days/wk. So week 1, I would work 2 days, coworker would work 3 days, and then the next week it would flip flop. OR, we could both come in for 2.5 days/week. But who really wants to come in for a half day on Wednesday afternoons? (I didn't)
4) I then wrote out this plan specific to my tasks/industry of how I could successfully transfer projects between myself and my coworker, etc.
5) We would still accrue vacation time as if we were one person (but just split 50/50) since we were both on the same level of vacation accruing.

I was actually VERY surprised at how open my boss was to this plan and the fact that it actually got approved. I guess if you never ask, you never know what they will say. The job share thing had NEVER been done at my work before so it was pretty cool how they were willing to take a chance on it. It was just a bummer that my circumstances changed and I ended up not being able to do it.

My advice is to go in with a good plan and to aim high.

Good luck!