A keyword expansion campaign should have only one goal – uncover untapped opportunity. Otherwise, you’re wasting time and money. A successful keyword manager, like a gold prospector in the American west, learns to recognize when a vein is tapped out and it’s time to move on to more fertile ground. There might be gold in them thar’ hills, but there might be a lot of fool’s gold too.
Learning to detect when a keyword is ripe for expansion is not a hard science. Here are some of the techniques SEO experts use to ferret out either ancillary or long-tail versions of a basic keyword.
Follow That Theme
The first tool in the arsenal of keyword research is to fully expand upon a theme. The idea is simple. Take a broad theme and work towards more specificity. An example can be seen in the keyword “pet food,” which is obviously a general term that would only be of value to a big box store like PetSmart. Unless you have deep pockets and penchant for self-abuse, don’t go down that road.
Start your expansion by thinking about all the different breeds of animals people keep as pets: dogs, cats, birds, hamsters, fish, snakes, etc. Add “food” to each one and you’ve expanded the idea of pet food far beyond the root term. 
Now let’s think about all the different kinds of dogs there are: German Shepherd, Shih Tzu, Doberman, Yorkie, Cocker Spaniel, etc. Obviously, that could go on for a while. Put “food” after each of these new terms. Move onto the next type of dog on the list and repeat the process. Already your keyword list is expanding by bounds and leaps.
At this point, you might want to call it good on the pet food theme. You could proceed to break German Shepherd down further like maybe German Shepherd puppy, old German Shepherd, but you have to wonder how much additional value you’re adding to your keyword list, especially when you could switch to an entirely new theme.
Find a New Theme
It’s time to do some lateral thinking. Food isn’t the only thing a pet might eat. What else is out there? For a start, you could throw in terms like treats, vitamins, nutrition, chow, fodder, feed. The idea is to pull out another top level term and then go through the expansion process already described. Your list will include dog treats, then German Shepherd treats.
This level of granularity is pretty easy to do with no other tool than the brain in your head, though you may decide that your time could be better spent on other activities. 
At that point, you might prefer to hand the grunt work off to a professional SEO team like the one at ManagedAdmin.com. They specialize in mixing expertise, elbow grease, math, and a little bit of magic to get your website onto Google’s first-page search results in as little as three months.
It wouldn’t hurt to spot check your expanded terms along the way because if they aren’t pulling any appreciable search volume, there’s no sense wasting your time trying to rank for them.
Good Guesses at Better Keywords
No one wakes up one morning an instant SEO expert. This stuff can be a tedious, maddening process that feels like the more keywords you reach for, the more escape your grasp. The reality is that you need to live the life, truly absorb it, and you’ll start to notice those intuitive leaps in logic becoming more commonplace. Here are some ideas to help you get there faster:
- Focus on Top Performers: This might seem obvious but you’d be surprised how many SEO masters, in the midst of keyword expansion madness, waste time working with root terms that don’t produce much value in the first place. Why?! If it’s not working at the root level, getting granular won’t help.
- Learn the Lingo: In order to brainstorm industry specific terms, you need to know the industry. That makes sense, right? There could be an ocean root terms out there if only you knew more about your client’s industry. How do you achieve that? Read, watch, and study, my friend.
- Get Competitive: It’s perfectly legal to look at which keywords competitors are ranking for and go after them also. There are all kinds of paid and free tools available that allow you to examine another website’s keyword focus. If they seem to be ranking well, get in there, mix it up, and try to outrank them.
The bottom line in keyword expansion research is to work smarter, not harder. Get an overall picture of what root terms are profitable, then break it down into narrower and new themes. At the same time, keep your eye on search volume. Do more of what works and less of what doesn’t. Now get out there and rip those root keywords to shreds.
For a deeper discussion of this topic, please read Amy Bishop’s article on The Ins and Outs of Keyword Research. The lady knows of what she speaks.