One thing to remember that no one has mentioned yet: When you are working with your horse, your end goal should NOT be to get her in the trailer.
Sounds strange, right?
I purchased clinton anderson's trailer loading DVD and it is awesome (highly recommend it). But if your horse doesn't want to put their whole body in the trailer, then you need to start small and gain respect.
This is more than just lunging your horse endlessly in circles. You should be able to move any part of your horse's body (shoulders or hindquarters, forward or backward) while you horse is on the end of the line, without you moving one step. THAT'S control and respect. So make sure you can do that first, before you bring the trailer into the mix.
work on this every single day for 10 to 15 minutes. Maybe the first day you will get her to load and unload one front foot. Great!!! Quit. Go do something else.
The next day, try to get both front feet to go in and out of the trailer. Great!! Quit. Go do something else.
And so forth. By asking/retreating over and over again, it becomes no big deal to the horse.