I'm a big advocate of reading about other campaigns to learn about how to run your own. In that vein, I recently read Game Change, which chronicled the 2008 presidential election. It's a great read if you're interested in how campaigns work, particularly at the presidential level. It is particularly helpful in understanding how candidate's messages are perceived by voters.
Back to the assertion that in every election, the incumbent defines the race. On page 64 of the book, the authors John Heilemann and Mark Halperin write:
So, how does that translate to your campaign?
Remember that voters will view your candidacy through the lens of the incumbent, even if you're running for an open seat and the incumbent isn't on the ballot.
When determining what the message of your campaign will be, be sure to consider what the voters know about and think about the incumbent. Then you can determine whether to contrast yourself with the incumbent or embrace him in a "don't change horses in midstream" style of campaign.