I only have experience in woodland caching, so you may have to modify some of this information for your region.
( picture from Google )
The most important thing that I can say about caching, make sure it is allowed in the recreational area. Also be sure that the container that you use will be picked up. My friend would always use 50cal ammo cans. They ran him around $13.00 and it guaranteed him that when he deposited it along side the road, it didn't stay there long. Other options have been food grade containers, but size tends to be the problem when digging the cache. Generally, mostly food items are stored in the watertight container as a resupply point. I'll run through a short list of what I have personally stored in some of my caches.
- Paperbook books - Thoreau, Orwell, Asimov , and short story fiction.
- Lighter - just in case and it takes up little room
- Candle for tent - old used tuna can for base
- Tuna/canned meats
- dehydrated meals - All individually Ziplocked, then Ziplocked in a gallon size freezer bag.
- Toilet paper roll
- Fresh socks - fresh socks are great!
- Capt. Morgan or Gentleman Jack
- Cigarettes - back when I smoked.
- Pasta/ dehydrated tomato paste/Parmesan cheese
- Navajo bread mix/small can Crisco
- Dried Fruit/Crackers
- Spare batteries
- Coffee/SUGAR/Tea Packets/Koolaid
- Map sections for further up the trail
- New Trash bag/ Wet Wipes
- Note - I'll explain later
LOCATION: It is usually located near a trail head or roadway for easy caching. I try to choose an elevated rocky hillside area to bury my cache. Make sure it is out of a flood area, where water collects, or pathways of water. IMPORTANT: You should very carefully consider where and how water will travel. Moss indicates constant water. A good location should be easily found by only you.
NUTS AND BOLTS: All of the items inside should be secure and stable. Once the container is closed, I like to sanitize the outside with a slight bleach water solution. I am probably being over cautious, but feral hogs and bear are a real threat to a hidden stash.
Security is also another issue. Do not bury the cache while other people are around.
Now, back to the contractor trash bags. The cache should be put into it with the opening of the trash bag pointing down. Think water flow. I'll dig a little deeper and place the fuel canister, also wrapped in a contractor trash bag, below the actual cache to keep any chance of contamination to near zero. The dirt is flung to make the area appear undisturbed. Any rocks are carefully set aside to be placed around and on top of the cache. Forest duff is placed around the area to help the cache blend into the area. I will almost always carry a few larger stones to place on top of the cache to mark, but more importantly deter any critters from getting into the cache.
The Note: Usually I leave a short worded note saying that I am a thru hiker and to please throw the trash away and thanks. Never signed, never giving direction, never giving any clues.