The art of getting UCE (Unsolicited Commercial Email) through the maze of spam filters changes daily. I have a couple of suggestions:
1) Make sure your content is legitimate and professional... people will be less likely to report your stuff as spam.
2) Use the SMTP server of your ISP to send the mail. Most ISPs do a lot of work to ensure that their servers are not blacklisted. One of the worst things you can do is send it from your own computer/workstation from a dynamic IP address (coffee shop, home cable modem), since that your computer is not in the family of known mail servers (mostly defined by special types of DNS records). Your ISP's mail server is likely able to handle a large volume of mail properly without causing issues with spam filtering/delivery.
This could of course bring you up against your ISP's terms of service, but if you're sending the mail as a legitimate business communication you should be fine. Check with their technical support before you send it to make sure you're doing it by the book.