Before going to sleep, try counting backwards from 100 to 1 by projecting an
imaginary white numeral onto a blackboard in your mind and see how far you get
before an interrupting thought invades the process. Eventually, you'll be able to do it
Next, recite mentally all the multiplication tables. Go over each product taking one
number at a time and recite its product. One times one is one through to one times
nine. Then two times one through two times nine, etc. See if you can go through the
entire multiplication table without having your mind wander off. The progress you
make can be easily marked by where you strayed off in the multiplication table.