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.