Non-doublers often seem to think that the most amazing thing about doubling is keeping all the fingerings straight. I don’t find that to be a major problem; the keywork of each instrument feels different enough in my hands that I think I tend to switch into the right fingering mode automatically.
It’s the other stuff that’s a problem. I find I often need to give myself a few reminders as I’m setting down one instrument and picking up the next. Here’s the stuff that has been going through my mind lately—maybe one or more of these will click for you, too.