Here’s a selection of websites that discuss how to write field notes:
http://www.ssc.wisc.edu/~jpiliavi/236/JOURNAL_FIELDNOTES.htm
http://www.sas.upenn.edu/anthro/CPIA/METHODS/Fieldnotes.html
http://www.anthropology.union.edu/fiji99/assignments/wk1notes.htm
http://www.home.earthlink.net/~mrance/IndexDocs/fieldnotes.html
This list is not exhaustive, simply those websites I’ve found useful and sprinkled liberally with commonsense.