Our memory of exact word sequences usually fades more quickly than our memory of (contextually interpreted) meanings.
More broadly, the exact auditory sensations normally fade very quickly; the corresponding word sequences fade a bit more slowly; and the interpreted meanings last longest.
These generalizations can be overcome to some extent if the sound or the text has especially memorable characteristics. (And the question of what "memorable" means in this context is interesting.)
Keep your contact information in a prominent location.
Write for yourself and for posterity.
Write pertinent field information with every new entry. You should enter the date, time, and location at the top of every page.
Add information on your location.
Record your methods.
Make backup copies.
If you use abbreviations, make sure there is a key in your field notebook.
Don’t leave home without it.
Form a writing habit. Thomas Jefferson was such an inveterate chronicler of daily events in his notebooks that he even took the time to record the weather four times on the day he helped write the Declaration of Independence. So unless you have something far more pressing than writing the Declaration of Independence, you have no excuse for avoiding your field notebook!
Set up a structure for your field notebook.
Create an index.
Treat your field notebook like a scrapbook. You should view your field notebook as a central clearinghouse for miscellaneous information that is relevant to your research project. If there are related bits of information that you will find useful later on, sketch them, write them down, photocopy them, and staple or tape them in your notebook.