Look for simple forms of percentages. Figuring 50 percent is simple, because it is half of an amount. In order to determine 10 percent, you merely move the decimal of the original number one place to the left.
Break a larger percentage into these simpler forms. For example, 30 percent off, is the same as 10 percent times three. So if you don't know what 30 percent of $12 is, you can determine that 10 percent of 12 is 1.2. Then you can multiply 3 times 12 to get 36. Reinsert the decimal to the left of the last digit and you have a $3.60 discount.
Round off numbers if necessary. For example, if an item is 29.99 and is 25 percent off, you can first round 29.99 to 30. Then think of 25 as half of 50. Since 50 percent of 30 would be 15, 25 percent is half of that amount. Half of 15 is $7.50.
Reverse the percent sentence to see which form is simpler. This concept follows the fact that x percent of y also equals y percent of x. For example, if you have the task of figuring 28 percent of 50, you can reverse this to figure out 50 percent of 28. The answer (which is 14 in this example) will be the same either way you calculate the problem in your head.