-Instead of staring at the edge of the trail, look forward and ahead.  
 You will ride straighter that way.
-You will notice that the middle of the trail is usually rutted.  This 
 may cause problems during turns if you go on the inside.  Instead, try 
 taking the outside line the whole turn.
-Some single tracks are too narrow and too hard to ride.  If that's the 
 case, don't risk a fall.  Walk it.
-If you ride a lot of single track, you might want to reduce the width of 
 the bar to reduce the chance of your hands hitting branches.  Also, 
 L-shaped barends help a lot.  
-Always wear eye protection.  You will need it.

Others add:
Bill Rod [smts!brod@msss.attmail.com]           
-Don't sightsee, your bike will go wherever you look. "Look at the tree, hit 
 the tree"
-If you are unable to clean an obstacle, get off and climb over it.  DO NOT 
 ride around the obstacle, thereby creating a braid.  This leads to  
 erosion and angry Park Managers, not to mention trail maintenance people.
-On twisty singletrack, try getting your butt an inch or two off the saddle. 
 This allows you to more easily use body english for maneuvering the bike.