With all the buzz about Social Media/Web 2.0, it's easy to forget about marketing to your customers. Social media is great to connect and provide customer service, but not so great for sales messages.
The area that needs further exploration is database marketing (aka CRM). Per AdAge.com, most companies are not doing this well.
It doesn't make sense to bombard your customers with thoughtless messages when you have the data to target them more appropriately. The best way to start is testing messages and response rates on a small sample of your data.
For example, I worked on a campaign targeting customers who bought a vehicle and paid an interest rate over 12%. Assuming they made their payments, they might qualify for a lower rate and a newer car. We targeted customers who had purchased three years prior, assuming they rebuilt their credit and now had equity in their vehicle. They were sent e-mail (if we had their address), a postcard and the sales staff followed up with phone calls. We found the customers were very transient and were difficult to locate. We then tested customers with the same criteria but who had purchased two years prior. The results were much better.
The key is to segment your data, run the campaign and measure the results. Once you analyze the results, you test again.
Business today is about using resources wisely. This applies to your customers and your marketing budgets.