There's also the method of using numbers that are already purposely in place.  For my "Math Cache" I used two signs placed by the Forest Service, one with historical information about the Arizona Trail and another showing a hiking trail, both containing numbers which were hinted at in the cache description. 
 
Jerry-Offtrail
 
 
On Thu, 28 Mar 2002 18:28:43 -0700 "Ken" <kenh199@hotmail.com> writes:
    Just a thought that I would like to post on some caches that I have run across. I have done several multi stage caches where the cache owners have written the coordinates for the next leg on items of public/private property. In my opinion this activity clearly fits the definition of criminal damage as described under Arizona's Revised Statutes A.R.S. 13-1602.
 
    I would like to make a suggestion to correct this trend and to possibly save fellow Geocachers from having to face possible criminal charges. If the coordinates for a multi stage cache must be listed on public/private property write it on a piece of tape and then place it on the object. This way if the property owner discovers the markings there is no permanent damage to the object and can be simply removed by peeling the tape off. Another alternative would be to use an inexpensive dyno-tape machine to imprint the coordinates on the tape and then post it to the object.
 
    I'm not trying to preach here but would like to keep my hobbie/addiction form getting a bad reputation. I know that if I discovered that someone had been writing on my property I would not be real happy :(  
 
    As a final warning while out placing caches this weekend our family spotted 3 snakes (and a mountain lion)  so be careful where you are putting your hands.
 
    Thanks for letting me air my concerns and this is not intended as an attack on anyone. I realize that the cache owners may not have realized that what they were doing may bring trouble to them and the geocaching community.
 
Ken