If you don't fancy the idea of carrying all the supplies for your entire trip, you can cache food and fuel at Longmire, Mowich Lake, and White River. Caches must be stored in rodent-proof containers, typically a 5 gallon plastic bucket. Food caches can be mailed or delivered by hand, while fuel caches must be hand-delivered. See https://www.nps.gov/mora/planyourvisit/caching-food-and-fuel.htm for details.

Note that caches for pickup at Mowich Lake should be delivered to the Carbon River Ranger Station instead.

The Longmire cache location is inside the Wilderness Information Center, which is only open between 7:30 am and 5:00 pm. Other locations are self-serve at any time.

Each cache location also has bearproof trashcans for disposing of your rubbish.

If you are camping at a cache location (Mowich or White River), consider including heavy treats that can be eaten on the spot. Tinned food, soda and beer ftw!

There are occasional stories of food being stolen from caches at Mowich Lake. This is very rare, but has happened more than zero times. Cache locations also have a donation bin where people can leave unwanted supplies, so I hope the backpackers who had their food taken were able to find enough replacements to keep going! And to whoever was so low as to steal from others in the wilderness: I pray to Our Lady The Mountain that you may be eaten by a bear forthwith :-)

Speaking of which: bears are common on Rainier, so it's important to properly hang all your food and other scented supplies such as toiletries. Thanks to bear poles at every camp, bear canisters are not needed during the main hiking season, but the poles are taken down during the off-season.