Seth Godin offers up a simple idea for keeping your email effective - don't just keep sending the same thing, or subtle variations on it. Do some new things to keep people interested - the metaphor he uses is crop rotation to keep a field fertile and productive.
I'd just add some caveats. First of all, don't send things that break your promise when people opted in to your list. I'm sure somewhere, a less than stellar marketer is saying, "See! Seth Godin says we should send a sales pitch to the people who signed up for our industry information newsletter!" No, no, no.
Second, don't stop sending something unless it's not working. If you are a lucky marketer who sends the same basic pitch out every time, and every time you get a great response, knock off work early today and bask in your success.
Finally, the idea is that your customers might be bored - not that you are. Don't change just because you're tired of what you're doing. If it works, keep doing it. Bad marketing ideas sometimes come about because the marketing staff is bored. ("Oh, I'm feeling such ennui... let's change our logo!")