I still have a lot to learn, but I like to go out right at sunset and get my telescope set up. Start getting the computerized mount set up and aligned to the first two stars I see while there is still enough daylight not to trip over it. It also gives the optics a chance to acclimate to the weather. Then I use the time waiting for it to get truly dark to pick my target and fine tune the alignment with 30 second single exposures (if I can see the target at all). I'm on a budget system though (alt-az computerized mount with webcam-style eyepiece camera and laptop). If you have a more permanent setup, a more capable mount, or otherwise faster setup, you may not need as much prep time as me.