Here is a good tribes ascend overall guide that was posted on the tribes ascend forums that includes a good picture about skiing.
http://img833.imageshack.us/img833/9012/shazbot4dummies.png
For skiing you want to be moving on a downward slope as much as possible. Otherwise you should be using your jet pack to get over a hill and then using your momentum to slide again down the vertical slope.
The higher you get in the air before you fall down and the bigger the slope, the faster you will go.
It's also worth noting that if you use your jets to "trim" or "adjust" your descent to land in a particular spot you'll be slowing down your effective rate of drop, which will in turn slow down your forward momentum once you land. When folks first start playing they think the fastest way to move is by "jetting forward", but it isn't--jetting to the TOP of a hill (not over it) is much, much faster because you spend time on the ground building up additional speed. The jets will only get you moving forward to a certain speed, after that it's all about forward inertia from the ground slope, so RIDE as much straight and downward slope as you possibly can!