The guys over at StreamSend posted a couple of tips on How To Reduce Your Spam Complaints by 75%. Here's a summary:

If you find that you are getting 1 complaint out of 1,000 emails sent or more, you risk getting your IP address blocked by one or more ISPs.

We have found that customers who place the unsubscribe link at the top of the email often reduces their complaints by about 75%. This can easily make the difference between getting blocked and not getting blocked.

Move the unsubscribe link to the top and make it very visible and very easy to find. You might even want to make the text bigger. Some senders might be concerned that a very easy-to-find unsubscribe link will reduce their list size. The fact is, people who want to get off your list are going to do so either by using the very easy-to-find spam complaint button (which can hurt your delivery) or they will click your very easy-to-find unsubscribe link.

By making your unsubscribe link highly visible, you keep your emails from being blocked from those who really want to receive them.

It might even be a good idea to put the unsubscribe link at the top regardless of your complaint levels. The ISPs have made it extremely easy to complain with a click of a button so you might as well make it easy to unsubscribe.

For some people, the "mark as spam" is the standard way they unsubscribe from lists rather than taking the time to scroll down to the unsubscribe link at the bottom of the email. If you have low complaint levels now, you could make them even lower.