How can I prevent my mail from being classified as spam?
Don't use a fake address, that's reeks of spam
Set up DomainKeys for your domain
Set up SPF for your domain
Put an unsubscribe link in your email
Additionally sometimes domains and IP addresses can often enter into blacklist databases which can cause emails to be triggered as spam or some email providers have been known to reject the emails completely. I recommend that you check both the IP and the domain against major email blacklisting databases, in the event that your IP fails, then I recommend that you request a new shared IP or purchased a dedicated IP.
First things first: are you sending spam (unsolicited mass email)? If you're not operating a strictly opt-in list that is carefully maintained, then you will end up spamming a lot of people, who will get annoyed with you and report you as a spammer (simply by clicking the "mark as spam" button).
If you're not spamming people, then you need to make sure you use an authenticated mail server that isn't being used by spammers. Also, setting up SPF as John Conde suggests will further verify that you're a legitimate mail sender since receivers can use reverse DNS to verify the sender is the owner of the domain the email is coming from.
There are also content checkers that try to see if your email contains any spammy text, but as long as you're not writing stuff like "FR3E VI4GR4!!! F4KE R013X" in your emails, this isn't a big issue these days.
Also, what volume of email are you sending? What is your opt-out policy? If you're on a large web host with a decent tech support, you could consult them and see what they suggest. Usually legit web hosts have a vested interest in not getting their IP blacklisted as a spam source.
In addition to the other suggestions, set up Reverse PTR for your mail server's IP address.