We've been taking pigs to the abattoir regularly for about five years. Ours live outside in electric fenced pens all their lives so we have no effective way of containing them in a small area for loading except by throwing together something temporary out of hurdles etc. Here are some observations I've made.
If you have say, 10 pigs, in a group, and you want to load 3, then only the smallest three will go into the trailer.
Remember to turn the leccy fence off before they get a shock off the trailer ramp.
Make sure that you can actually drive away once loaded and are not stuck in the mud.
A porker will go where he wants to, despite anything you put in his way to stop him.
If you get stressed the pigs will sense this and will go out of their way to make your job harder.
All the above advice about feeding in the trailer is good but remember you'll always have the odd stressful loading no matter how many times you've done it. All this pales into insignificance compared to some of the scenes in the abattoir yard when the b******s won't come out of the trailer.