If I were asked to choose out of all the aspects of marketing including online marketing, marketing strategy, market research, etc.
, what I consider to be the most important element, I would say: "testing".
This may sound surprising as there are so many factors to creating an effective campaign such as: getting the right message across; reaching the right audience; getting the timing right, with the right offer; but if you think about it, this is the very reason why testing is so important.
These very elements can be identified and optimised using proper testing.
  By testing email and SMS campaigns, multiple landing pages, direct mail packages, keyword campaigns, and other marketing campaigns, you can find what resonates best with your audience and set proper expectations for performance.
 Once you've established a baseline, continue to test and experiment with different messaging, timing, and offers and compare your results against your baseline.
 After testing you can confidently define how well your campaign performed, be it good, bad, or indifferent based on real data.
One area I recommend strongly on testing prior to sending out an email campaign is subject lines.
After designing an email campaign, create three or four different subject lines.
Keep everything else the same (the email creative, the price, the timing, etc.
) and send it out to a small number of your database and you can be sure that at least one subject line will outperform the others.
Once you have identified which, it will make a huge difference to your overall campaign performance when you send it to the remainder of your database.
Ongoing measurement and testing is essential for marketing success.
  If you're not testing your marketing campaigns on a continuous basis, you are more than likely missing out.
  My recommendation: continue to test, test, and test some more and you'll see your knowledge and results increase.