Simplest is to use the WGS84 Datum, same as the satellites, and a UTM grid to provide Eastings and Northings.
Within your SatNav, there should be something in Setup that allows you to choose the Datum and Grid.
Note UTM works in 6 degree longitude Zones starting from 180West to 174West with North and South Zones, then 174West to 168West etc. If you are near a Zone boundary like Greenwich Meridian then select which UTM Zone you want to use and stick with it for your caving area. Make sure everybody's SatNav is set the same and check them all at the start of the expedition and at the end. Once they have all settled down with good satellite availability and been positioning for a while, you should see agreement better than 20m for the same location.
If you are in a country with easy access to good quality maps then try to use the same Datum and Grid parameters as the country mapping, which is usually detailed within the map legend. Here in UK, set your SatNav to OSGB Datum (also called OSGB36) and OS Grid.
Note your magnetic compass will most likely require an adjustment to agree with the grid North. This usually consists of two parts. First one is to correct your compass to Magnetic North. Second is to correct your Magnetic Compass to Grid North called Declination. Be very careful of the correction signs you use in Survex. If in doubt, process without any corrections, note the position of the point furthest from the fixed point. Then process with the corrections and see how the co-ordinates have changed for the point furthest from the fixed point - has it rotated correctly ? Always list your Datum, Grid, correction and Declination terms in the Survex file.
That is about as un-geeky and simple as I can make it.
Hope it helps.
The BCRA Cave Surveying booklet is a good reference.
