Get onto the ISP's feedback loops for spam complaints - and action those complaints (basically, if someone hits 'report spam' instead of unsubscribe then don't email them again) - details of the big ISPs are here: http://blog.wordtothewise.com/ISP-information/
Make sure bounced emails are removed from your mailing lists. We use a neat piece of software called bbounce but googling for it I think they've pulled it now - was owned by the Whitaker Firm LLC
We looked into Goodmail's certified email program which provides whitemail listing for Yahoo and AOL (and some others) but they weren't interested! (our total email volume is less than 50,000/month and we currently use an ESP not on their list). I had a very helpful conversation with a sales guy at StrongMail who thought Goodmail was probably overkill for our volumes.
Plus make sure that your emails have the location address of the organisation/sender in the email and an easy to follow unsubscribe link. I swear I've read somewhere industry insiders reporting that the content of emails are checked for this sort of content and factor into the deliverability equation - if you're not already, check out Mark Brownlow's excellent No Man is an Iland website.