I've done it a couple of different ways.
After backing the boat most of the way into the water, tie off the dock lines with enough slack to allow the boat to float off when backing the rest of the way in (you'll have to get the boat properly tied off before pulling the trailer out of the water).
The other way is to use an extra long bowline. After stretching out the tow strap (but before backing into the water), take the bow line all the way to the tow vehicle (tie it off on the hitch with just a little slack). This will keep the boat on the trailer after it's floating. After the boat is floating, untie the line from the vehicle and walk it back to the boat and use both a bow & stern line to pull it off the trailer.
The first method worked fine when there was no other traffic around, and the 2nd works better when the ramp is busy.