When I first introduced my yearlings to the trailer, I had tied them to it one at a time for grooming every day for a week. After grooming by it for a week, I untied them and just walked them right in like they knew how to load (one at a time, obviously!). Latte walked right in with me, and Frappe hesitated at first but followed me in with a gentle tug on her lead.
The reason everything went without fireworks? Groundwork was firmly established, the trailer itself was not a new concept, and I didn't act like it was a big deal. Compare it to backing a horse for the first time. If all the groundwork is established first and you've taken things slowly, there shouldn't be any fireworks when you take the next step and swing your leg over. Same goes for trailering.