Much has been written on the topic of keyword research by other SEOs. I, myself have written extensively on the topic of research and selecting the best keywords for an optimization campaign. Without trying to restate anything that has already been written, (at least by me) let me present a few ideas that can help you identify all of the possible keyword phrases that you could benefit from.
Over the years, our keyword selection process has become quite refined. Looking over the research we've done in years past, most is good, but doesn’t live up to the 5-phase keyword research and selection process we use today. The first and most important step in our research process is to identify the site's major core terms. If you've read through the previous series of articles linked above you'll note that our process has evolved slightly since then. Identifying core terms (what we called “keyword themes”) used to come later in our research process but we've found that by knowing all the main and/or relevant core terms, you’re better able to find all the most important phrases.
As you identify your main core terms, keep in mind that you are looking for unique two- or three-word combinations, not actual search phrases. For example "tahoe wedding" might be the core term which will cover numerous phrases including "lake tahoe wedding". Similarly "vow renewal" covers "wedding vow renewal" phrases, "wedding favors" covers "wedding shower favors" and so on. Usually we'll combine plural and singular variations into a single core term but sometimes we'll separate them, depending on the circumstance.
We begin by pouring through the site, scanning title tags, keyword tags, description tags, text, navigation links, products, etc. By doing this thoroughly you'll be able to identify all of the main site core terms which have already been established. Looking through products and product descriptions will produce a gold mine of core terms. Keep in mind that we are looking for unique two- or three-word phrases. If you're looking at motorcycle battery products, for example, you might pull 'motorcycle battery' as your obvious main core term, but you’ll also find that 'honda battery', 'solar battery' and '12v battery' fit under the 'motorcycle battery' core term. These also will produce a number of unique phrases you can target that don't necessarily contain the word 'motorcycle.’ During the course of your in-depth research finding actual searched phrases, many keywords will overlap across multiple core terms. You're better off duplicating than you are missing any.
Once you've worked through your site, do the same with a competitor's website. Perhaps they have used a unique word combination you hadn't thought of. After that it's time to jump to a few tools to dig up even more core terms. I've heard good things about Keyword Discovery but as of yet I have not had a chance to try it out. I continue to use (and love) WordTracker, especially their new Keyword Researcher tool which they have been developing. I've offered a few suggested improvements so far and a few of them have already been implemented.
Useful in developing my core terms, Wordtracker has a related keywords search tool. Type anything in here and the results produced below are other phrases that may (or may not) be relevant. Pick through those results and document any new core terms found that are not already on your list. Google and Yahoo both have tools which will provide similar information allowing you to find such core terms, and I also use L3xicon.com quite extensively.
Running a keyword search through L3xicon provides results showing related words, definitions and even related web pages. It's the related words we are most concerned about, and these results come in two sections, both can provide useful information
Running a search for "tahoe wedding" L3xicon produces some results that allow us to find some additional core terms that may not have already been added to our list:
wedding chapel, wedding package, wedding planner, wedding ceremony, wedding service, wedding planner/planning
These may have already been discovered when pouring through the site and through competitor's sites, but perhaps not. Next we're going to try 'outdoor wedding', a phrase that probably came up in our WordTracker search. Here a few more relevant core terms pulled from the results:
garden wedding, wedding location
Now let's click on the 'garden wedding' link provided and see the results. Look! Another core term we can use:
Let's try going back and searching for 'wedding location'. From this list of results we are able to pull:
unique wedding, romantic wedding, exotic wedding, destination wedding,
And the searches can continue until you have finally found what you believe to be a complete list. What then do we do with this list? We send it to our clients for review to ensure that we have not included core terms that are not relevant. For example our client in Tahoe may not provide any type of garden themed weddings so they'll let us know that this phrase is not relevant to their business. After that you can begin the more in-depth research of finding all the related and actively searched phrases for each of these core terms, another time-consuming but rewarding task in the long-run.