Keep the board flat on the box. Thats really all there is to it. If you start sliding off the side of a box be patient...
For sure. And I think someone else mentioned the "don't look down at your feet". Looking down at your feet is an almost surefire way to end up on your ass. When you commit to the feature, look beyond the end of the feature. I know the instinct is to look down and make sure your footing is correct especially when starting out with boardslides, noseslides, etc., but you just have to trust yourself
Keep practicing on the smaller boxes or rails after a few sessions you will really get a feel for what it feels like when you are about to slip off early, or what it feels like when you don't "lock" on the feature. At first you are going to try and fight this feeling and try to balance it out but this almost never
works. Once you know what it feels like, you should be able to smoothly decommit yourself and ride away unscathed. That's board control that comes with practice but you'll get it if you keep working on it!