In real estate, its location, location, location! In SEO it is research, research, research!
The first step in any significant SEO project is to determine what keywords you are trying to optimize. Even if you think you have a pretty clear idea, it is always worth spending a little extra time up front researching and planning your approach.
Why? Because all of this SEO stuff takes time, so you might as well get it right the first time. Plus, you may not be as aware as you think you are of all of the various terms that people are searching for. There are tools available to help you with this, so there is no reason not to take advantage of them. You may be surprised at what people are actually searching for, and there is no sense in missing out on opportunities.
One place you can go (it actually may be the best place) is to Google Adwords program. The "keyword" tool they supply can tell you most of what you need to know when you are trying to decide what words to focus on.