Neil Anuskiewicz wrote a great blog post on avoiding spam filters. Here are some of the tips he shares. Read the rest on his blog:

1. Unless you really have made a breakthrough such as cold fusion
or a flying car, avoid claiming you have made an important
"breakthrough!" Filters love that.

2. Do not talk too much about
cash on the barrel head. Excessive discussion of money is like manna
from heaven to many spam filters. If your topic is money, well, you
have to talk about it but try to be discreet and not directly mention
money too much. You know what the "get rich quick scheme" emails look like. Avoid the "get rich quick" look and feel at all cost.

3.
Sorry Mortgage brokers but if the email looks like a mortgage pitch,
the spam filters are likely to latch onto it. There are many legitimate
mortgage broker marketers out there, of course, but the ones who abused
email in the past have hurt your current opportunity to get into the
Inbox. Make your pitch in more subtle ways, when possible.

5. A "money back guarantee" is sort of
like the "check is in the mail." Nobody believes it in any context
including email and the spam filters don’t believe it either.

9. Try to avoid sloppy HTML in your emails
and never send an email out that is basically one giant image. The spam
filters often assume spammers are sloppy with their HTML and legitimate
email marketers are likely to be more careful. Second, a legitimate
email marketer is very unlikely to send an email that is one giant
image.

Source: Streamsend