The worst single thing you can do on a box is hit it too slow, this gives you time to really try and hurt yourself on the feature.
As everyone has said aim off the end of box, as you do setup carves to get on the thing, imgaine the carve lining up your landing on the other side instead of lining up the front of the feature.
Sometimes it is the simplest things.
I've been working on boxes and learned a few tips from here and a coach at my local hill which have been covered here. I tried to practice the idea that once on the box look to the end to ride it all the way. I always struggled with approaching the box, trying to be lined up straight to ride the length but my eyes were always on the beginning of the box. Once on, I'd shift my view to the end of the feature. Unfortunately, many times I'd be just a few degrees off and would go off the side near the end or before making it that far. It never dawned on me to look at the end BEFORE getting on the feature. This simple suggestion makes so much sense, and is so obvious I'm ticked I haven't thought of this before.
And this is why I love this place.