As a picture is worth a thousand words, use this great little diagram to ensure you account for the magnetic declination on your map when practicing your land nav skills. If you don’t trust your map, you can always go to http://www.magnetic-declination.com and get the exact declination for the area you’re in. Personally, as I have a real ‘trust’ issue with the net and anything man made, I take the two, and if they’re not the same, I split the difference. Example: The map I use for my training area indicates a 9 degree Westerly magnetic declination. The web site with up to date information says 7 degrees West. I use 8 degrees West. I’ve found that to be more accurate; YMMV.
You can get something similar to this at http://www.maptools.com, or if you’d like a more robust field reference that’s waterproof and includes a hell of a lot more information, check out our DTG Field Reference here. It makes a great Christmas gift!