Tristan, you should look at the previous discussions at Serverfault.com -- a site much like this one, but targeted at system administrators.
In short, there is no way for you to say "Hey, I'm not spam! Don't filter me.". If there was, then spammers would abuse this system too (sic).
There is a war going on between spammers and good people, and sometimes innocents get hit. What constitutes a correct answer today may not be entirely correct tomorrow.
Having said that, here is a terse list of what I consider the most important things to do right now:
Use separate outgoing mail servers for bulk email and business email (you stated that you don't do bulk email, so this is not relevant for you).
Make absolutely sure that each outgoing mail server has a static IP address, has its SMTP HELO string configured correctly, and that reverse DNS lookup to the IP address will return its HELO address. (If you exclusively use professional mail providers (such as Google Apps) then this is already handled for you.)
Set up Sender Policy Framework for your domain (SPF), i.e. whitelist which servers are allowed to send email on your behalf. It's not that hard to do; if you use multiple email providers then you just need to include each providers SPF record into your own. Here is Google's Apps for Domains SPF info. SPF is something you set up in your own DNS records.
Write good email body text, i.e. use the recipients name prominently (maybe both in the subject and first line of the body), don't send pure HTML emails (if you use HTML, include a plain ASCII version too), don't use spammy language (ALL CAPS, MORE !!!!!!'s, certain words).
Make sure the "unsubscribe" link or mechanism for opting in / opting out of communication is easy to find and easy to use. If people can't unsubscribe when they want to, they'll just hit the "This is spam" button in their email client, and your online reputation will suffer.
This one is debatable: Send from a real address on your domain, which you actually monitor & respond to. This is to catch complaints from the (total ******) users who only know how to operate the "reply" button in their email client, and / or can't be bothered to actually press a "unsubscribe" link. Of course, providing good customer service is always valuable.
There is also a fine standard for strong email sender authentication called DomainKeys. It's a good standard, a few large email recipients support it (notably Yahoo! mail), so I'm not down on it at all. But right now it's pretty complicated to set up by yourself and support is not that widespread, so I don't consider it a "must do".
Real, honest statistics about email delivery success rates are hard to come by. Most of what I have seen was commissioned by the email deliverability industry itself, and therefore possibly biased -- and certainly some of it seemed alarmist to me. A fair guess is that if you do the above, then 'more than 98%' of your emails will be delivered successfully.
For peace of mind, you can from time to time check a online resource like MXToolbox's blacklist tester to verify that you're not blacklisted anywhere. This is for each outgoing mail server IP address, so if you use a large provider like Google for outgoing mail, then don't bother -- they already do this for their mail servers.
