Here is a great blog from Kristen Gregory about testing your email campaigns.
9 Tips for Testing Success

Want to make sure you’re conducting the best tests you can? Keep these tips in mind:

1. First, always run tests more than once. Flukes happen. You can’t try something one time and be certain of anything. I recommend running a test AT LEAST three times (if not more!) to determine an actual trend.

2. Stick to one test variable. When you test more than one element, results get fuzzy and which element really made an impact becomes more difficult to determine.

3. Make sure that you are testing segments of your list that are large enough to make the test valid. If you’re sending to a list of 300 people, how valid is a 5% test segment? As a rule, I say go larger with test segments if the list is on the smaller side. The larger the test segment, the more valid the results will be.

4. Focus on the right – or all – data points to determine winners. Typically, subject line tests are designed to drive opens, though they can drive clicks by being more call-to-action-oriented. If boosting revenue is your ultimate goal, make sure you’re looking at the full picture.

5. Give the test enough time to run before declaring a winner. If you send out test segments at 9 am, you probably don’t want to declare a winner by 10 am and send that creative out to the remainder of your list. You need to be sure that subscribers have had enough time to see, open and interact with your messages.  Allow as much time as you think would make sense, and, at absolute minimum, a few hours. Recognize that a test segment that looked like it was “losing” when you declared a winner can pull ahead in the end. Always take a second look when more time has passed.

6. Be smart about WHEN you send test segments. I’ve seen some marketers send test segments at 2 am and the winning creative at 7 am. I would argue that unless you’re assured that your audience is comprised of early risers with plenty of email-browsing time, this isn’t the best way to test.

7. Choose the right kind of split test. Some platforms, like Bronto, enable marketers to send tests to a smaller portion of a list and then automatically send out the winning version (based on most opens, clicks or conversions) to the remainder of the list later that day or the next. This is a really fantastic way to optimize within a single send. At the same time, I believe some testing works best if you let it run its course and evaluate afterward. For example, when evaluating major creative changes or offer options, I think it’s more valuable to see the full results (rather than exposing a small test segment for a small amount of time) and be certain you’re making the right choices for the future.

8. Always be testing! Make testing an integral part of your everyday program. If nothing else, run a simple subject line test each time you send.

9. Actually look at results and adjust future tactics. Make sure to review what’s working best so you can play off of lessons learned and continue to improve.