Guidelines for using a GPS for XC flights
Firstly, regarding the grid refs I need for your xc entry, we're talking about OS National grid refs, ie. in the format AB 123 456. For more information see the Ordnance Survey website.
To get the grid refs off your GPS if you have one (and let's assume it's a Garmin) you'll first need to check in the Navigation Setup page that the Position Format is set to "British Grid", and most importantly, that the Map Datum is set to "Ord Srvy GB". Is this is not done then any OS grid refs you give will be wrong when plotting them on an OS Landranger map. However if you're flying in a comp abroad say, and have turnpoints supplied in Lat and Long format, then you'll need to set the Map Datum to "WGS84" (remember to set it back to "Ord Srvy GB" when you return to the UK), and set the Position Format to the appropriate Lat and Long setting.
When using the GPS in-flight, what I do is press the GOTO button twice before I take off - this sets the MOB (Man Over Board) position to be the take-off position, then when you're flying you'll always have a distance (and bearing) to MOB displayed on the moving map screen showing you exactly how far you've flown so far. When you land, press the MARK button to save your landing position.
Then when you want to get the co-ords off the GPS, go into the Nearest Wpts or Waypoint List pages and select MOB and your LZ waypoints and read off the OS grid ref.
NB. The grid ref stored here will be in the format AB 12345 67890, which gives the position accurate to 1m. When giving the co-ords to me I'm only interested in your position accurate to the nearest 100m, so round the figures up or down accordingly - eg. in the example above it would be AB 123 679.
I also upload a database of towns and large villages to my GPS to help with in-flight navigation (my GPS38 doesn't have any built-in mapping database), but that's beyond the scope of this "tutorial"... (I'll be adding info about this information soon!)
See also Ulf's GPS page - VERY USEFUL!