A lot of the "Natural Horsemanship" trainers (Parelli, Monty Roberts, Brannamen, Tom or Bill Dorrence, Mike Peace, Kelly Marks, Mark Rashid, Pony Boy, etc etc) have done good work about trailer loading. Most of the NH trainers have a different "cosmetic" look, but teach approximately the same underlying concepts. If you've found one that you like, then I'd recommend looking up what that trainer has to say about trailer loading.
This list is not complete by any means; there are many out there who have their own approach to training, and most of them are pretty good. Not everyone "clicks" with any one kind of approach, so it's important to find the one that's right for you.
Trailer loading can be easy if your horse trusts you wholeheartedly, is willing to follow you into any space (even if it is narrow or dark), and the horse has no previous experiences to overcome. Unfortunately, a lot of horses have had bad experiences or simply do not entirely trust their owners. This is why I recommend finding information on NH; most NH methods start out with exercises to build your horse's trust in you.
For my difficult loader, I used join-up (the term comes from Monty Roberts' work). My mare was bossy and didn't want to let anyone else tell her what to do when she was scared. She'd also had a pretty unpleasant trailering experience, and so was uncooperative. Once I had established that I was a courageous and trustworthy leader, she never questioned loading onto a trailer again. The very first time I asked her to load after our join-up session, she was a little hesitant about the footing (she'd previously been trailered in a step-up, and this trailer had a ramp). After she'd figured out how to get on, she loaded right on; she has had no hesitation since.
Join-up worked for me and my mare. No one approach is "one size fits all," and something may or may not work for you--or for your horse. You'll need to find something that works for both of you
Once you do, though, I'm sure your relationship will be revolutionized. Easily loading onto a trailer is only one terrific benefit of a good relationship with your horse. You also get better ground manners (the horse respects you more), you can get less spooking under saddle (if you are confident, the horse trusts you to know what you are doing) and so on--it's just amazing what a really solid, trusting relationship can do for a person and her horse.
When I was at a show with my mare she wouldn't trailer and 4 hours later we sudated her and pushed her in. ONe of the trainers there said we should put her food into the trailer but have water to the side and she will soon go in and learn the trailer is a good thing. I haven't tryed it yet because we have had alot of rain but maybe it will help you out.