Don't know how well I can describe this, but I'll have a go.
Park your box/trailer within inches of (and parallel to) a long straight surface. A wall for preference, but a solid straight fence would do, or even another box.
If you're loading to the right of your box, have the wall on the right, with your box positioned so that the right-hand 'run-out' area is now a wall (or solid fence, or another box).
(If you're loading to the left of your box, park facing the other way, so left-hand 'run-out' covered.)
If you have an assistant (assuming right-hand load) lead forward, with your horse's off-side close to the wall (or solid fence/other box). Go steadily and quietly and confidently, fair leading pressure _ good momentum, but don't pull or jerk. Assistant comes in to the left/rear of your horse with a dressage whip to tap-tap-guide, thus stopping the left-hand 'run-out', (in extreme cases assistant can use the bristle end of a brush, to encourage & guide - not hit).
Without an assistant (and in many ways I've found it easier without an assistant) you lead from the left with lead rope in your right hand (wall to your horses right) with dressage whip in your left hand holding slightly behind your left side to tap-tap-guide left/rear of your horse.
I don't know if I'm explaining this very well - it's harder to describe than I expected - but I have never had this fail, and have used it with several different animals & helped others use this too. It's always been quiet, calm and successful. Even with a two-year old coming 'off-the-field' (it's owner's euphemism for I-haven't-bothered-to-handle-it-enough)!
Also, I should add that it might sound a bit of a bind finding a suitable wall (or other), but I've always managed to find a suitable place with relative ease.
Good luck!