I've had great success playing the parelli games with my my. Use a rope halter and lead along with a carrot stick( or lunge whip). Present the trailer. If your horse exhibits positive signs let her be, but if she backs up put pressure on her to drive her back towards the trailer. Release pressure immediately as soon as she steps towards the trailer. You can find videos from Pat and Linda Parelli that explains this process better.
If a little food tempting hasn't worked, here's something I used on a horse that REFUSED to step foot on a trailer. Have the trailer set up in an open space, and have you're horse on a lungeline near it. Present the trailer to your horse (DO NOT FORCE HER IN). If she's actively checking out the inside, let her. If she's backing away or disinterested, bring her away from the trailer and start working her in the lunge line at a trot/canter. After awhile of lunging re-present the trailer to her RIGHT AFTER lunging. Being by the trailer should be her reward. If she's advancing in the trailer, and checking it out, she rests. As soon as she looses interest, or backs away. Bring her away from it and lunge her again. Repeat this process until she is hopping straight into the trailer without the hassle. This might take some time, so be patient and keep at it. This process has worked wonders for me.