I'm not much for close-in draft for safety concerns. This past year, I saw a big rig kick-up a pallet board into the driver's area of a Toyota minivan. Everyone was fine, but it looked scary
On the highway, I usually rely on the momentary "side-draft" or pulling action of the tractor-trailer. If I'm loading-up a hill in the right lane, a middle lane truck can generally be found making it up the same hill, but faster.
When they pass, there's some initial wind blocking that continues to the rear trailer wheels and (for me) seems to taper-off at about 4 car lengths behind the trailer on either side. This helps get up the hill. If they're going pretty fast, they can pull a bulk of air with them and essentially pull you up the hill -- although slight.
The same idea can be used if you're being consistently passed by other drivers.
Just don't hang out too long -- it can irritate the driver or create a blind spot.