I never had the problem of forgetting to keep peddling when i came up to something, because i remembered about the fixed.
It was when i was stood up and tried to freewheel for a second or something that the problem came.
When i had been riding fixed for a while then went geared i tried to leg brake, once, but that was it.
I can go onto geared bikes from fixed, then back onto fixed no problems, or at least i could do.
I do end up sitting in one gear though, or having 50/19 as my smallest gear.