Sorry Bez, not disagreeing to disagree, I hate to "bait" a trailer with food or water or treats to get a horse to load. It doesn't make a horse feel safe in a trailer, it just says water is in the cave and I'm not thirsty.
I watched a Clinton Anderson siminar a few years ago and tried his method on a few of my horses, it works great. First make sure the trailer is hooked to the truck because they do bounce in and out of it. Set the trailer somewhere with plenty of room so you can longe the horse behind the open trailer. In a nut shell you longe the horse behind the open trailer (with a longe line, not a lead rope and secure,tie, the doors open) pushing the horse with the whip, don't hit the horse, hit the ground behind the horse, when she is moving away from the trailer and taking the pressure off when she is going toward the trailer. So, when you're longing your filly it's an unpleasant time when she is making the half circle away from the trailer and easy time whenever she is going toward the trailer. After a few minutes of this, as she is going toward the trailer while longing, you step toward the trailer as if you want her to run right in... she won't but let her stop and be as curious as she wants, smell the trailer or what ever, for as long as she is curious. As soon as she stops being curious about the trailer and looks away move her off easy until she is moving away from the trailer (in the same direction you always go in) and put pressure on her again until she goes half way around and let up as soon as she starts the circle toward the trailer. Keep this up she will soon get the idea the trailer must be the safe area because nothing bad happens when I go toward it and pay attention to it. Pretty soon she is pawing at it, stepping on it and putting feet in it. Don't spook her at the trailer let her take her own time getting feet on on off, just remember to push her going away from the trailer when she is not interested in the trailer anymore, again don't put pressure on her when you move her off the trailer because she is no longer curious, wait until she is moving away from the trailer. Make sure the floor of the trailer is not wet or slick because all of my horse have ran and jumped into the trailer, not joking, and one went down in the trailer, taking the training back many days, try to keep them in a trot going toward the trailer. When they do get all the way in make sure you reward her, BE SAFE, but when you feel comfortable and safe get in the trailer with her and pet her and ease her mind, start looping the line threw a tie down in the trailer, don't tie her until she is comfortable with it. It may take 1 to 1 1/2 hours the first day but after a few days you should be able to lead her in and tie her safely and comfortably.
Many ways to train a horse to trailer this way is easy on the horse and you and gives you quaility time with your horse. And it is amazing to watch it work. I have never tried it in a straight load, only a slant, but AI see no reason it wouldn't work in a straight load and even be safer for someone getting in with a nervous horse. Remember to be safe when you get in and reward her, turn her head towards you (if you are using a slant load) so she can't kick you or back over you.
Let me know if I can clearify anything for you, it's kind of hard to explain if you hav enever seen it done, but a lot of natural horseman use this type of method.
Good luck,
Alan