We have a dolly to lift the front. Car gets driven in while someone steers. My other buddy uses a remote winch with the cable attached to the rear and dollies his into the trailer. The dolly also duplicates as a front end jack.
When we had the truck and trailer we built a step ramp system. Pull up, put the ramps behind the rear tire and back up on to the ramp. Then we could roll the car in and out.
Like some have mentioned, a nice long flap door 4-6 feet and the tall rubber bumpers will help a lot. Adding a little air in the rears always help and so does running the front of the trailer up some.