My tactic would be very similar to what Arab123 suggested. Work her (energetically) outside the trailer, then give her the chance to get on and rest. NO REST UNLESS IT IS ON THE TRAILER. If she chooses to fuss and not get on, start lunging again. When she makes the connection between resting and the trailer, let her rest in there. If she starts to get out by herself, let her, then work her again before giving her the chance to get on and rest again. My mom and I saw Clinton Anderson use this method in a demo at Equine Affaire with a horse that nearly refused to go in the same arena as the trailer, and had the horse literally begging to be on within an hour. While feeding can get the horse on the trailer, at the end of the day it is a bribe and just covers up the problem. Anyway, unless your mare is very food motivated, she'll probably get sly to it anyway.
Sheer curiosity, but how tall is your trailer? Since we have cattle, and not enough money to buy a separate trailer for the cows and the horses, we bought a stock trailer. Since our horses are both under 15 hands, we figured they'd be fine in the shorter trailer. Wrong. After a month with the new trailer, one refused to load without a ramp, and the other wouldn't go any farther than putting his front feet on, then flying off backwards. We ended up raising the trailer roof by nearly a foot, a little higher than the standard horse trailer. Both horses instantly loaded like a dream. Trailer even looks better, lol.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown