There are several conversion optimization testing methods you can use. One of the most common is the A/B test, known as one of the split tests.
In this process, your original web page (or email address) is called a "control". You change a single element called a "variable" to create a new version of the page or email. Then you split your traffic or audience so that half of your traffic sees each version. At the end of your test (we'll talk about test time later in the article), the version with the most conversions wins.
You can also run A/B/n tests with multiple variables and split traffic evenly between them. Most professional conversion rate optimization experts recommend no more than 3 or 4 variables for any given test.
A third option is to toggle multiple options on the page at once. This is called multivariate testing. Measuring with this test is more difficult, however, because you can't be completely sure which of the changes you've made leads to higher conversions. Then there's usability testing, where you get real users to take tasks on your site and see how well they're able to accomplish them.
Other than usability testing, these types of testing often require a fairly large audience to produce statistically significant results. If you're just starting out, you may not have enough traffic or email lists to properly practice A/B testing. In this case, it's best to measure the results against your goals before and after a change, and also listen carefully to user feedback.