I have had good luck with making the trailer the place where they are fed but I did have a horse that didn't eat for 3 days before I gave up and fed her outside the trailer. We ended up resorting to pretty harsh methods. A trainer (not me) used a stud chain across her gums so everytime she backed up it put pressure on her gums while someone else drove her from behind with a whip. It took about 15 minuites and she was in the trailer. That is a last resort method.
Remember they have to be able to go down the road too. Once the horse is loading well take a few short trips. I start by going up and down the driveway (about 200 feet) and work my way up to around the block (that would be 4 miles in country blocks!) A lot of people make the mistake of thinking that once the horse will load the work is over. Getting them comfortable going down the road is just as important. I ride with a horse new to a trailer just to make sure they don't freak out too badly. Driving GENTLY
and not forgetting that you have your beloved horses behind you helps too. No slamming on the brakes and taking corners slowly.
I want to add that we did try the lunging many many times for days and she still wouldnt go in. She would go into any other trailer but not the one we had. It was a dark color and an average size 2 horse straight load. I never could figure out what it was about that trailer she was afraid of. We have since sold the horse and the trailer :)