A little more tips for getting more comfy with boxes and rails: In addition to what Wolf said, a) Try to always pop up onto a box or rail, even if it is a ride-on feature because you always carry a subtle edge when on snow but on the box u wanna be dead flat, b) Always make sure your weight is over the board i.e. shoulders, hips, head etc should be over the board at all times, c) When you get onto the feature always remember to bend your knees - noobs always just freeze their lower body to keep their feet flat but keep their knees locked straight, making you very unstable and unable to react to anything sudden, d) Always look at or past the end of the feature. Its very important because it helps u make it to the end. Lots of people go off sideways because their line of sight is wrong. Then you end up trying to correct on the box, which only leads to disaster and e) Always pop off the box and absorb the landing with your knees. Lots of people who lock their knees straight cant do this. They bring the same frozen knees to their landing, and end up falling.
In sum: Body weight over board, bend knees, eyes on the prize.
Boardsliding and Back Edge: There is always a natural tendency to lean back when one is unsure or not confident. You need to first of all avoid this and project your weight at a 90 degree angle down at the box. i.e. stay perpendicular. Some boxes are sloped downhill gently and when approached from front look like they are dead flat but when you ride past the feature and look from the side, you can see they are not. If you scope out the feature before hand, it will help you in figuring out how much you need to lean. When you are doing backside board slides and are facing the end of the box, try to be conscious that your toe edge (yes the "downhill" edge) needs to actually stay in contact with the box to maintain perpendicularity and be dead flat. Avoid leaning back and lifting your toes. Similarly for frontboards (i.e. sliding backwards), lots of people favor their toe edge and lift their heels. You should keep both edges dead flat against the box meaning that this time your heel edge, which is the "downhill" edge needs to stay in contact with the box. Once you get your edges right, be mindful of your body position. Keep your weight stacked over the board. Bend your knees and do not break at the waist, which is another classic reasons for boardslide failure.
In Sum: A dead flat base at all times, adjust for feature inclination, no breaking at the waist.
Hope this helps a bit.