-Elbows relaxed, bend at about 90 degrees.
-Grip the bar firmly, but not too hard.  If you see white knuckles, then 
 you are gripping too tight.
-Keep your back straight, at about 45 degrees from the ground surface.
-Try to "stand" on the pedals.  You still sit on the seat, but you 
 don't place all your weight on it.  
-When not pedalling, always keep your pedals level.

 Others added:
 dmerson@ksu.ksu.edu
-You may want to add that your grips should be about shoulder length apart,
 but that would only really affect small/large people.
-You could mention that you can (or may have to) ride with pedals
vertical
 (&/or with one foot loose) in tight turns.

richard@prl.research.philips.com
Many people spin their pedals slower than is optimum. Faster feet in a
lower gear will often give you more speed and less fatigue, although it
can take some time to get used to spinning your legs ar 90+ rpm.