How do I avoid SPAM filters to get html newsletters through to people who have signed up for our newsletter?

As SPAM filters get ever more stricter, I'm seeing my newsletter open rates drop dramitically. I know that newsletters I sign up for end up in my junk mail box all the time. I know how to make them a 'trusted' sender but not everybody does. I'd love to be pointed towards free resources on the web which will give me a better understanding of how SPAM filters work and how to legitimutley get around them. This can include words or phrases not to use or any advice on how to educate the recipients on how to allow access to emails they've signed up for. Any help would be grately appreciated.