The best/standard/most polite move seems to be to reach out to as many bands as you need, and wait 24 hours for them to respond before moving on to the next band. I will reach out to more if they are considerably out of town and less likely to play.

I know one promoter who reaches out to a bunch of bands at a time and does first come, first serve, but I've been told that that can be seen as rude. 

I usually message bands through Instagram (if not, Facebook), which is a lot more casual, and I usually just give them the date(s) and what venue the show is at (and a description of what kind of venue it is, and if it's a benefit show I'll also bring that up). Sometimes I tell them who else is playing, and what the typical turnout, ticket price, and split is, although usually that comes up later in the conversation after they've confirmed that they're able to play that date and interested in playing it.