The Eight Rules of Field Recording:
1) Always slate your recordings at the top.
2) Use windscreens.
3) Always strive for the healthiest signals (pre-amps can help).
4) Record at the highest fidelity (sample rate/processing power) available.
5) When possible, vary proximity to the recording subject with alternate takes.
6) Utilize headphones in the field.
7) Bring plenty of batteries and/or charge up before heading out. (Make sure that media cards have plenty of memory, too.)
8) Naming conventions: determine a clear naming scheme for your collected recordings.
The two biggest challenges to field recording will always be wind noise and signal gain (healthy recorded signals). Learning how to work with these challenges will be the key to getting consistently good recordings.