Stole this from Harry Soo and his review on amazon for seasoning your new pit.
The key step once your smoker arrives is to ensure you season it properly. New WSMs will tend to overheat until you are able to get grease and gunk on the inside and around the rim where the lid sits to ensure an air tight seal. The fastest way to season is to do the following: Phase One - cover the water pan completely with aluminum foil and run a full load of lit Kingsford briquettes and let it run as hot as it can with no water in the pan (over 350 degrees) to burn off any manufacturing residue. Clean out the ash and proceed to phase two.
Phase Two - fill the charcoal basket 1/2 full of unlit briquettes. Then put in a 1/2 chimney of lit Kingsford briquettes in the middle. This will allow a slow burn for 3-5 hours at 72 degrees outside temperature. Adjust the vents to get 275 degrees on the dome thermometer. Put bacon strips, chicken parts, pork fat, or any other scrap meat you don't plan to eat. The key is to get fatty meats to generate lots of grease. Toss in a couple of tennis-sized wood chunks to generate smoke. Repeat Phase Two at least twice before you cook meat that you want to eat.
Phase Three - When you cook meat you plan to eat, take a tip from me and don't use any water in the pan. When I cook, I just cover the water pan with foil top and bottom. I foil it twice so I can remove the second layer after the cook and refoil it. That way, I don't have to clean my pan. It works just as well, AFTER YOU SEASON YOUR WSM, when you cook without water in the pan. Dry heat allows the crust to form faster on the meat (called the bark). Once the crust forms on the meat, you can introduce moisture. I just spray water with a regular spray bottle to encourage bark formation after the initial crusting (Maillard reaction) has begun. To test for properly formed bark, use your finger nail and scrape the meat surface. If the crust has formed, it will not come off when you gently scrape it with your fingernail. If the crust comes off, the bark has not set (still wet) so don't spray until it sets. Let it cook longer and check back in 15 mins. You'll get much better results this way. We've won many awards with this technique.
When it comes to cleaning your WSM, never wash the insides. Get a good grill brush and scrape down the insides and dome. You need the "aroma" which takes several years to build up. I NEVER mix my meat WSMs from my seafood and hotdog WSMs. Nothing destroys the aroma faster than cooking fish/seafood/hotdogs in a WSM used to cook chicken, ribs, pork, brisket, and tri tip. That's why you should buy a pair if you plan to cook seafood/fish/hotdogs. Better yet, get a Weber Kettle for those meats. Remember to always empty the ash from the bottom and grease on the foiled water pan to avoid fires and any rancid old oil smell before you cook. When you need to clean the grates, put the grates in a big plastic trash bag, put on gloves, and spray oven cleaner on the grates while in the bag and let sit for 1/2 hour. Hose off the grates. It's as easy as that. To clean the outsides, I use Simple Green spray.
Enjoy your WSMs. They are awesome and built to last.