1. Never ever do black hat SEO. Black hat SEO involves blasting your site to thousands of directories, blogs, forums, ect... This is a sure way of losing your rankings.
Note: Your rank may actually boost for a little while but once Google and other search engines pick up on your tricks, you will lose your rankings altogether.
2. Don't just pay anyone to do your SEO work. Most SEO companies have bad reputations and may use black hat methods to try and rank you highly. Do a lot of research if you plan on hiring a 3rd party company. A lot of people lost their rankings when they hired 3rd party companies. Be careful.
3. Want to buy 10,000 visitors for $10? Don't! That kind of garbage traffic doesn't convert anyway. In fact, this will harm your rankings. Why? Google will see that most people who end up on your site don't stay longer for a few seconds. 99.9% of garbage traffic is just that. The traffic is usually a bot that goes to your site, stays on for a few seconds, and leaves. This harms your site and makes search engines think your site is worthless since nobody stays longer than a few seconds and nobody browses other pages on your site.
4. Do not use link farms. Link farms are one of the worst things you can do to your site. Some companies will offer you easy ways to get back links if you put their code somewhere on your site. What this does is show Google that you are promoting thousands of outbound links that are promoting back to you. Its unnatural and does not work. Google considers this spam and devalues your site real quick.
5. Having broken pages, broken images, and broken links on your site will hurt it. If you don't upkeep your site, Google will notice and devalue it real quick. Google has not patience for webmasters that keep half working sites while the other half is broken. Always check around and make sure that your site is in good working order. If 95% - 99% of the site works well, you will be ok, but anything less than 95% will harm your site.