I do this all the time with cake mix and pancake mix. You can typically make the mix and store it for quite a while as long as the wet ingredients are separate. Months in fact. Keep it in an airtight container. I keep my individual batches of cake mixes in ziploc bags so i can just dump them and add the ingredients. I would recommend this for muffins. For pancake mix, I use a big airtight plastic container.

amylou321, I thought you were going to say that you add the wet ingredients directly to the ziploc bag containing the dry mix, close the bag, and squeeze to mix it up! That's not a bad idea, actually. :o)

Sorry I'm so late to this discussion (been away for Canadian Thanksgiving with family and to visit our granddaughter at college), but I've been making my own "convenience" foods, including muffin mixes, for many years. Depending on the ingredients, they will last anywhere from 3-6 months at room temperature, and you can double that time if you store them in the freezer. When I made these mixes with freshly-milled wheat (or other grain) flour, I always stored them in the freezer and used them quickly - within a month or two. Although wheat flour will keep well in the freezer, the nutrients degrade quickly after the flour has been milled.

You can find all kinds of "in-a-jar" recipes that might work well for you. These are often stored in a quart jar, but a quart-size storage bag or a quart-size plastic container will work just as well. If you go to Amazon.com and do a search for "Gifts In-a-Jar" cookbooks, they have a muffin and bread one. I just gave my set of these books to my niece (no longer useful now that we're gluten-free). You can actually make any of your favorite muffin recipes a mix by using the same method.

Did you check your local library for books on the subject? For many years I used "Make-A-Mix Cookery" and "More Make-A-Mix Cookery" by Karine Eliason, Nevada Harward & Madeline Westover.

Another way to use a mix-type method is to make a homemade baking mix (Bisquick-type mix). This mix has a multitude of uses, including the base for muffins, and Bisquick was actually one of the first convenience foods. The link below is to Utah Ready-Quick Mix, a publication by Utah State University Cooperative Extension.

The one mix you should avoid are refrigerator bran muffins where the mixture is left in the refrigerator. These recipes are no longer considered safe to use due to the raw egg in the mixture, where they are stored for a prolonged period of time in the refrigerator. They have a higher potential for bacteria growth.

-Grainlady

Here is a link that might be useful: Utah Ready-Quick Mix