I like to spray my truck with the foam gun before I even get out the buckets, etc. I don't particularly care if it even dries up a bit, 'cause I give it a second soak anyway. I always shake up the 50/50 soap/water mix in the foam gun and go for max suds. By the time I'm ready to start washing, I give it a flood rinse, and suds her up again with the gun. Now the only dirt left on the truck is a layer of surface film that's pre-lubed with the foam. When I get around to the front end, the bugs just float away with a light swipe.
I'm not one of those guys that think a foam gun is magic, but it sure helps in getting the truck ready to actually put a mitt against it.
Another thing I've learned here is to do everything in a back and forth motion...not circular.
With the above foam gun method and lots of soap in a bucket, and a sheepskin mitt, it's so slick when you wash you'll think you're floating the mitt over an air cushion.
I guess the point here is that letting even a mild soap dwell for a while takes a lot of work out of the actual physical part of the wash.