i usually put wood, or skis on the back side to keep the snow in one spot. The salt you use is the same stuff they use to melt snow on sidewalks. YOur grocery store may have it, but your better off at a hardware store, cheaper. make your hit in layers. build like a 6' high jump, then salt it down. Put a little bit of salt down. too much and you will cause too much melt. THe idea is you melt it a little, and then it re-freezes slower, making it harder, it becomes more like ice than snow. then apply another layer, then another, and keep applying a little bit of salt on it. By the time you get a decent size jump, salt down the whole thing, all sides of it, but remember a little bit goes a long way.
If it is done right, it will stay hard as fuck, and it will last a long time.
Spinks