I guess you can tell I am both cheap and old-school by the fact that I use a broom. I already have the broom anyway to sweep out the trailer and whatever.
When it's time to hook up I just lean the broom against the coupler on the driver's side. I then back up using the rearview mirror (the one on the windshield) and line up the center of the high-mounted center brake light
with the broom handle (well, a little to the passenger's side because the broom is on the driver's side of the hitch) then back up until the broom is nudged.
The ball is in the right place every time.