the way you described is the way iv always done it. and if there's a better, faster, easier way i don't know about it.
iv also tried doing them first by masking off the outer edges with 1/8 fine line tape. then masking off the body and painting the trim. for some of the higher detailed tamiya body's you almost have do it that way because the window mask just doesn't cut it. but on 99% of the body's out there it's not necessary. and doing it that way on say something like an truggy or buggy body wouldn't get you any better results, it would just be a waste of tape and time.
do it last like your planing on trying. it's a lot easier than you think it is and the margin for error is greater than you think. a line has to be pretty far off to be noticeable. your biggest fear is cutting through the body or the paint bleeding. using a new sharp blade is the secret. makes the job a lot easier. and feel free to take a sharpe or pencil and draw your line on the window mask so you've got a line to follow. that also makes it easier because you're tracing a line rather than having to eyeball freehand it on the fly