Cognitive psychologists say that some things should be so automatic that you shouldn't even have to think about it. Factors through 20 (at least) are one of them.
We have gotten away from memorizing some things, but how can students do higher-order thinking without some background and some memorization of facts...
My students love review cards. Cards are printed with question on front and answers on the back - print and cut. The students pair off and go through them together for 3 minutes each day. A quiz is given after the 4th day. It becomes automatic to them. I have used them in middle school and high school.
With this purchase you get:
*set 1 with factors through 10
*set 2 with factors 11-20
*a quiz for each set
*answer key for each quiz