1. STOP what you're doing. Consider SEO before you start building anything. It's much easier to do SEO if you build the site around your strategy.
2. Do thorough research into your market and select your keywords carefully, considering how competitive terms are, not just their volume.
3. Don't be tempted by what's best described as "SEO Tricks". It's unlikely you're going to out fox Google, and a ban is the more likely result.
4. Make sure your on page SEO doesn't stand out like a sore thumb. Content should still be readable and if it's immediately obvious what keyword you're optimising for, chances are you've got it wrong.
5. That old chestnut, but still very true. Create original and high quality content. Your visitors and the search engines will love you for this more than anything else you could do.