What kind of lubrication should I use for a lock?
OLD ANSWER (Improved below)


  Graphite powder is the preferred lubricant for locks.  You should be
  able to get it at any hardware store in a squeeze bottle that is half
  air, allowing you to blow it right into the keyway.  You are going to
  have to wait a while before putting it on, as the residual WD-40 will
  gum it up.


EDIT: As per MrSquonk's comment below -- Try coating the key and work it in slowly.  It's less messy.



LATE EDIT:  Comment below edited into the answer.  Please read full comment thread.

Yale USA says: 


  Yale® KeyMark® cylinders are lubricated from the factory with a
  Teflon® lubrication. Cylinders should be lubricated periodically
  depending upon environmental conditions and usage. LAB Lube is the
  approved lubricant. Caution: It is not recommended to lubricate
  cylinders with oil or to mix lubricants. "


LAB Lube 


Micronized polytetrafluoroethylene (PTFE) Powder 
A dry white powder lubricant that will not "cake-up" as graphite does. 
A superior friction-fighting agent