Over time, all contact lists, even those composed of the most pristine opted-in volunteers, will accumulate inactive records. Properly handling these inactive subscribers is one of the most important ways to keep your communications program strong.
Stefan Pollard describes the correct maintenance steps to take and avoid below, but I'd also like to emphasize a painful issue that is really the most common: boredom. The only way to keep an audience is to keep them entertained, and surprise is an important requirement. No matter how relevant the information is, there are only so many times we can hear it repeated. The pressure is on you to keep your audience awake. David Ogilvy said you can't bore anyone into buying your product.
ClickZ: The Right Way to Trim Inactives, 2008-Sep-24, by Stefan Pollard of Strongmail
The first and most important step in any reactivation campaign is identifying your audience. I recommend segmenting your audiences using the metric or factor you use to define inactivity, but don't change anything right away.
Send the same creative to your inactive segment that your active subscribers receive. Make sure your inactives truly don't respond. This provides the opportunity to refine your segmentation should you not properly identify the inactives on your first attempt.
Before you trim the deadwood, try to awaken inactives from their slumber with a well-planned reactivation campaign. Reactivation doesn't mean you can approach long-ignored addresses or unsubscribes. This approach can backfire and drive spam-complaint volumes up to the point where ISPs will block the e-mails you send to the truly active subscribers on your list.
A clear message telling readers what benefits they can get as subscribers should be the centerpiece of your reactivation campaign. Sending out a pouty or poignant e-mail saying you miss them doesn't do this. ... Instead, remember your normal message strategy isn't working, and you'll need to try several different and new approaches to reach your inactive subscribers ... (Read the whole article for his specific suggestions.)