For Channel cats I like to use creek shiners. Seine the creek by the house, put them on ice, and head out. I usually cut them into nice bite size pieces and roll them down infront of a good logjam or at the head of a deep hole. It usually doesnt take long and it seems that we get the bigger fish on these compared to using stink bait.
For flatheads theres nothing better than a small bullhead, 4-6 inches. I like them better just because it seems they never die in the water or in the bait bucket.