the deeper the ramp, the easier it usually is to load. i would think you want to be able to at least float the boat 1/3-1/2 way up the trailer, or more, if you can before cranking.
some other things to so in no particular order:
1. unplug your lights before you back it into the water.
2. make sure you don't back your trailer wheels off the end of the ramp. have someone watching if you can.
3. i have a two wheel drive so i make sure my rear wheels don't go into the water so they don't slip on the algae and seaweed. four wheel you may be able to go deeper, but you don't want your differental on the rear axle under water.
4. if you are pulling it out with the mast up, check for wires etc all the way to where you are going to park to drop it.
5. remember to take something to wear on your feet in case you have to get wet. i also like to crank out the strap before backing up as it makes it easier.
6. if your rudder is fixed, you may want to remove it so it doesn't hit the ramp on the way out.
7. you may need a long stern line if your ramp has a short dock in order to get it back far enough to back in the trailer.
8. remember to take the mast crutch if the mast isn't down. also, you may want a step ladder and some tools: needlenose pliers, wrenches for turnbuckles ect, hammer and punch for tabernacle pin. plenty of rope, bungees etc to tie things down.