Begin with a short, straight rail just a few inches off the snow. Approach the rail straight on, with moderate speed, knees slightly bent, skis shoulder width apart. Tip: If you've recently had your ski edges sharpened, they could be too grabby. If so, detune the area directly underfoot.
Hop up on the rail while turning both your shoulders and skis 90 degrees in one fluid motion. The biggest mistake people make is not rotating far enough—not getting their skis completely perpendicular to the rail—which is the surest way to get tossed.
Keep your skis flat and your eyes glued to the end of the rail—not the spot where you're going to land. This helps keep your body aligned and balanced. Crouch down low for more "steez" (new-school jargon for style, but you knew that.)
When you hit the rail, make sure you're fully committed, with your weight distributed evenly over the middle of both skis. If your weight is too far forward or, more commonly, mostly on the uphill ski, you'll get sent.
Lots of people lose their balance trying to "pop" off the rail. It's easier to relax and just let your skis slide off. Turn your shoulders back downhill as you do so and your skis will follow.