Once you are on the ground...
Once you are on the ground and sliding, try to orient yourself on your back, traveling feet-first. This will let you see where you are going as well as spread the weight of your body over as much an area as possible.
Once you've initially hit the ground, the object is to do whatever possible to avoid starting to tumble. While it's generally good to keep the outer portion of your limbs from flailing about, you need to try and spread yourself out in order to avert tumbling; the more you are tucked into a ball, the more likely you are to tumble-which will almost assuredly result in broken bones and prolonging of the actual fall itself. If possible, orient yourself so you are sliding on your back, hopefully feet first. Your back protector (you are wearing a back protector, aren't you?) makes a nice wide, flat surface to spread the load over, as well as protecting you from localized hot spots and road rash, and the more surface area you can drag on the pavement, the more you'll scrub off speed and the quicker you'll come to a stop. The important part to remember through all this is to remain as relaxed as possible, while still moving parts of your body to avoid additional injury. For instance, curbing at the edge of the racetrack is easy to catch something on, and you want to "surf" over this area as smoothly as you can; by slightly lifting whichever limb is at the forefront of your slide, you can avoid catching something and starting a tumble. Sound impossible? You'd be surprised at what you can accomplish sliding along the asphalt, and how much that effort can save you from pain later on.
Another big mistake made by first-time fallers is to try and get up while they are still moving. This is another recipe for tumbling, and it's worth being certain that you are stopped before attempting to move. A good policy is to count to ten after you think you have stopped, and be sure the crash is over. If you're on the road or racing surface you'll want to extricate yourself if possible; otherwise, take inventory of your body and if anything is unusually painful, wait for help to arrive.
Once the crash is over, evaluate...
Once the crash is over, evaluate what happened. Not only do you want to consider the cause of the fall, but also what you could have done differently during the crash.
Even if you avoid serious injury in a crash, there is always the stiffness and soreness to deal with during the days after. Alternate ice and heat on especially hurtful bits, and stretch regularly to avoid having your muscles tighten up. Of course, go to the hospital or see your doctor if something seems amiss. Most importantly, learn from your experience-hopefully you won't have to put that knowledge to use again, but it doesn't hurt to be prepared.