Possibly the most important aspect of promoting your website is to know what terms you want it promoting for. A mistake often made is to optimise just the homepage and then for a couple of the most obvious search terms. The best practice is to keep building pages all optimised for different search terms. There is usually less competition for the lesser search terms and because they tend to be more specific, they usually convert better. Google has a useful and free tool for getting an idea of what search terms to use based on search levels. The keyword tool can be found here.
Below is a video from Matt Cutts on how keyword research is hugely important:
