Make sure the results are statistically relevant. One or two responses are not enough to tell you whether one test succeeded over another. You should try to get at least 50 to 100 responses for each test before you can make broad judgments about which performed best.

You can arrive at the number of recipients you'll need to send to by using your average clickthrough rate. If you have a list of 10,000 recipients, and your average clickthrough rate is five percent, then you could expect to receive 500 responses. So to get 50 responses in a test, or one-tenth of that total, you'll need to test one-tenth of your list, or 1,000 recipients.

Next: Maintain a control group