BigManDan,
I use everything I can! I have a shortbed pickup truck, and when I get to aiming at the ramp I get out and lower the tailgate, don't do it sooner to make sure to not hit it with the boat/trailer (been there done that, ouch).
I cannot leave my neck turned backwards for very long, so depend on the mirrors mostly, with the tailgate down the rear-view is a big help. To quote Red Green... "...you watch it in the mirrors, or actually turn your head and look if you are under 25..."
I leave the rig in neutral and let gravity work to do down the ramp. As others give different tips, you can pick and choose what fits for you. Practice time is good and kind of fun unless too many people are watching
The biggest problem I have now is "finding" the ramp again when you are on the flat and the trailer is on the slant and more or less below the horizon..
My son at 16 helps as a spotter. He remembers the times when he was very young when I could spot it in one try, and is kind of amused to watch what happens now!