For touring around, at least. Mine's an eTrex Vista, this is how I use it:
First get some decent software. I use SeeYou, but there are others. Make sure you get 3-D views etc - that's what you want it for, to see yourself spiraling up in some great thermal. 
Then set your waypoints to be meaningful landmarks for you - LZ's, take off points, your next XC distance target, mountain peaks. Digitise them and get them onto the GPS. Don't load 100's otherwise you'll have a hell of a job selecting them in the air... give them meaningful names, too, not just "102AZX". 
Make sure you have the track set to a level that will work for you. I guess you have around a 10,000 point memory, so either set it for a few seconds (3 or 4) or just auto (that's what I use for my eTrex Vista). On auto 3 hours of XC seems to be around 5,000 points.
Waypoints: put LZs in at their mean sea level (MSL) height (or more correctly WGS84 ellipsoidal height, there can be quite a difference in some places). I fly mountainous areas, so I put my other waypoints mostly as peaks, so again I use MSL. This means that when I make a valley crossing to a peak, I have the glide angle required to reach that peak. I guess that for flying in the plains, one would add a standard margin above the way point but I don't have that experience.
I use the map display as a thermal tracker, so I set the zoom to be 120m or so, then I can see the 360's - and judge if I fall out of the back, wind direction, drift, etc. Turn off the "snap to roads" so the display is normal. I set the map display so up screen is the direction I fly (in other words, the screen turns if I do). This seems much more natural - it is a flight instrument, not a map.
For XC flight, I switch to the "trip computer" display, which I have set up with the most meaningful fields: ground speed, glide angle, glide angle to destination, altitude, time to destination, distance to destination, destination name, etc. I can't remember all, but skip through the choice list and you'll get the idea. I use the glide angle to destination to decide if I go on glide as I'm climbing in a thermal (I also have this displayed on the map screen), and to judge if I am winning or losing on the glide.
In flight I generally choose my next waypoint depending upon the conditions, clouds, etc. This means its easier to use the favourites list for the waypoints, or choose from the "nearest" function. You need some practice to do this. The 76S will be better than the eTrex because the buttons are bigger.
Make sure you have it working, the memory clean, and the track recording, during your pre-flight check. Then after you land, stop the track and then switch off. When I get home, I download via G7toWin (even though SeeYou can read directly) and save the IGC file. SeeYou is then able to do all the stats (flight time, etc) automatically and you have nothing else to do but sit back in an armchair and enjoy your flight...
First posted by me here on the Oz forum, Feb 2004
3:07:12 PM 
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