You've got the right idea. No matter what, you're gonna get dirts in it. The $10k SprayBake booth we have at work gets dirts with me blowing myself off, putting on a spray suit, and spraying a single piece on a horse, not even attached to the vehicle... Still get dirts. Bottom line is just do everything you can to prevent it, it's gonna happen but the more effort you put ino preventing it the more minimal the problem of dirt nibs is gonna be.
Also, if you're doing this in your garage, be sure to remove any shine products such as ArmorAll, any tire shine, anything greasy really. They'll cause fisheyes even if they are across the room. Damp floors, as you mentioned, definitely help. There's an enormous difference between spraying on a wet floor and a dry one. The simple act of walking around kicks up dust from the floor even if you can't see it and where's it gonna go? You got it, right in your damn paint job since it's wet and/or sticky.
I know I've got a TON of things to add to the "how to paint" thread but hopefully that helped you out some.
EDIT: Also, yes, clean the living snots out of any jamb that you're gonna be spraying across. Clean the entire thing, not just the edges. Jambs, corners, pockets, that's all areas for shit to sit in and it doesn't take much of that shit to blow out to nuke a paint job. With clear it sucks because then you've got a lot more to sand/buff but in your case...well...you know.
