Mobile Crossing did a piece the other day on waypoint averaging (see their February 7, 2006 entry), and it struck me that many readers might not know about this. The idea is that your GPS may only be delivering an accuracy of a certain number of feet, but if your GPS can take multiple readings and average them, you can get more accurate waypoint placement.
Many receivers, including my Garmin 60CS, have this function built into them. You can set your GPS down at a trail junction and let it take a number of readings. The UseNet group sci.geo.satellite-nav elaborated on this in a recent thread, discussing doing waypoint averaging across multiple outings and the use of GarTrip software to properly "weight" readings from multiple trips.