For the plumbing contractor I get the sinks, faucets, tubs (if not already installed), toilets, the things people will see when the house is finished (See Step 17 - Plumbing Rough-In.) As I've written before, the plumber can/will do this for me and make this part of their scope, but I don't want/need them to shop for me. They don't know my taste and they won't be in a position to snatch up a sale item they see that's similar (but maybe not exact) to what I specified. I buy what's needed and deliver it to each room so it's ready to go on the scheduled start day for trim-out. The plumber appreciates this, but it also allows me to double check myself and make sure I've got everything I'm responsible for.
I get ready for the electrician in the same way; by getting the light fixtures, ceiling fans, light bulbs, smoke alarms, door bell, etc. This contractor supplies the light switches, receptacles, cover trim plates, disconnects boxes, and everything to do with the breaker box (See Step 19 - Elec. R/I). Same as with the plumbing crew; I stage everything for the electrical contractor in each room since I'm the one who knows where things are supposed to go, not them.
And finally, there are a few things that the plumber and electrician will need to work together on at trim-out time. I supply these things too; the water heater, the garbage disposal, and the dishwasher are three examples. This is another reason that I like to schedule these two tradespeople to do their final bit of work simultaneously if possible. These items/appliances need water and electricity to work and unless the house is a micro-cottage, there will be plenty of room for them both at the same time. Even if they're a day or two apart, things will be fresh in their minds if you have to get them on the phone together to share information. However, when possible have these two trades trim-out at the end of the job at the same time.
Jump to Step 32 - HVAC Start-Up