Here is what you need:
-2 two by fours
-a piece of flat wood that is at least 1' by 2' and .5 in thick
-skate board trucks and wheels.
-a piece of pvc pipe
-Make sure the trucks are not much further apart than they would be on a skateboard. if they are handling and speed will not be very good.
-make sure the front wheels are closer to your body than your feet. this gives it better handling. the best weight distribution has the majority of the weight on the front wheels (this improves speed) so keeping the front wheels close to your body also improves speed.
-lubricate the wheels before you use it each time with wd-40.
-long board wheels or roller blade wheels work better than the wheels on most cheap skate boards so use them when available.
-braking with your shoes will wear the souls down really fast. use old shoes when using the luge.