I've been a park designer for 5 years. Here's the deal:
Salt lowers the freezing point of water to -8 celcius. Salt should only be used when the temperature is above freezing point and the snow is wet/slushy. When salt is added to slush, it lowers its freezing point, causing it to melt very rapidly. As it changes state, it steals energy from the snow beneath. the snow beneath turns to ice as a result, hence you have a faster, tougher surface.
Fertilizer has the same effect, but is more environmentally friendly.
Use lots! It's hard to really use too much. The important thing is to let the snow set for at least 15 minutes after you throw salt on it. If you ski on it right after you salt, you'll break the bonding of the particles and the snow will turn to sugar which is not good.
If you're doing this on your front lawn, you might want to use fertilizer... Hey, your parents might even pay for it then!