Well... I think to accept donations you need to apply to be a non profit, but I don't really know how that works. That will have the benefit of making your feed and supplies tax free, and you can accept donations and do things like raffles to raise money. Of course, then you will need to do book keeping and file a tax return for the organization, but it may not be too much of a headache with a small scale organization like yours.
You'll also need to plan to have volunteer(s) or employee(s) to help out if you're not around, whether you go out of town or happen to get sick or injured.
Being in Missouri, you also need to have back up plans for disasters. I don't know what part you are in. I know I constantly hear about floods, tornadoes, etc, in that part of the country. So, if an evacuation was necessary you need to have a plan in place (just like for pets and family).
You will want to have goals and write down to the rules of what you can and can't take in, and adoption procedures. Some animals you get in, as far as injured animals, you won't be able to help unless you are a veterinarian or have one that you can take them to (legally you can't do surgeries and such, obviously). Some will not be able to be released, so you will need to be accommodate that, space wise, or have somewhere you can send them.
In any case, you have to be willing and able to operate as at a loss - that is, fund out of your own pocket. If for whatever reason you don't have any donations coming in then you can't just not feed them for that month, you know?