I always use up the last bit of cereal at the bottom of the box that others in the family seem to pass up, by adding it to my container of "base" cereal, such as corn or bran flakes. The container is big enough to tumble the ingredients every now and then to keep things mixed up, and accommodate the contents of a new box of base cereal when getting low.
I also add raisins, nuts, granola, coconut, dried fruit, whatever. I prefer healthy cereals, but if I end up with a few Froot Loops in the mix once in a while, that's ok, too. I enjoy not wasting money. Also, I keep an extra 1 cup scoop right in the container for measuring portions easily.