The Most Important Thing
These all came from the Groundspeak forum "The single most valuable thing I can tell you about Geocaching is the following". These are not mine! I have not credited the authors here because you can get that information on the forum. I've just reproduced them here as a service for those who are new to geocaching and might not want to have to dig in the forums for this information. I do encourage the use of the forums, though.
What's REALLY the most important
Geocaching is like smoking...Once you start, it's hard to quit. Don't start!
Always bring extra batteries.
Mark a waypoint for your car.
Take a compass.
Bring a print out of the cache page with you.
Carry spare parts (Pencils, small notebooks, Ziplock bags) to help a cache in need of repair.
In the summer months, don't forget to bring your favorite DEET product.
Use the bathroom before you go caching... or bring TP just in case.
I know some people will probably disagree with this, but planning is EVERYTHING. Try to work out an average of how long it takes you to walk a certain distance (with help of your GPS that is) and figure how much time it will take you on average to find a cache, and throw in an extra five or 10 minutes or so to allow for rough terrain, and/or adjust accordingly due to the terrain and difficulty ratings on the cache page. This allows you to figure out how much time you'll probably spend at a cache, so you know how many caches you can hit in a day, maximize your daily cache finds...
ALWAYS take water.
Trade even or up.
Make sure somebody knows where you are going
Bring a plastic shopping bag with you so you can "Trash Out"!
If you cache in any wooded areas, shower as soon as you get home! You can usually avoid chigger bites and rid yourself of all ticks that way. Don't wait until 6 hours later to shower.
Print out the explanation of "What is Geocaching" from the web page and have it with you in pocket or purse. It will lend creditability to your attempts to verbally explain what you were doing if stopped by an officer of the law while on a hunt.
If there are people around, be careful not to expose the location of the cache to them. Come back later if you need to.
Double check the cache coordinates, especially if your hiding a cache.
Resist the temptation of sticking your bare hand into a hollow tree or hole in the ground if you can't see what's in there. Probe with a stick or wear leather gloves.
Beware of the poison ivy.
Make sure your GPSr is set to use the correct datum for the coordinates - typically WGS84.
Bring a First Aid kit.
Always have a spare vehicle key!