What are people looking for on your site? These terms are keywords in the vocabulary of your actual audience and should definitely be added to your keyword list. Do not forget to look at the keywords people filled in that didn’t get any results: this was stuff people were expecting but didn’t find. You can look into the results of your internal search in Google Analytics at Behavior → Site Search → Search Terms.
Use the Google Adwords Keyword Planner to find new and related keywords, but ignore the search volume data! The search volume data in the planner is really only useful for keywords that you’re actually spending money to advertise on. Otherwise, these volumes are not reliable. While not really helpful to decide which keyword is most used by your potential audience, Google Adwords Keyword Planner makes a useful tool in coming up with ideas for potential keywords!
I also want to emphasize that Microsoft Excel and Google Docs perhaps play an equal, if not more important role in keyword research than any of these tools, because at the end of the day, the mountain of data you end up with is of no use if you can’t perform calculations, sort, filter, present or store it in a way that’s best suits the task at hand. Head over to Distilled for a comprehensive Excel for SEOs guide. Alternatively, bug this guy—he’s the Sensei who trains Excel ninjas in the dark of the night.
I also hang out in SEO forums and groups and see KC scores tossed around and talked about and pretty much everybody is on the same page. Most agree that you can’t completely rely on the KC score (there are a few factors that can’t quantitatively be measured with a number) – but that the KC score is a VERY good indicator of the competition level for specific keywords. The following diagram realistically shows exactly what to expect from certain KC scores…
I was going to post screenshots here showing my numbers for the past year – but the dashboards in the old affiliate platforms (Clickbank and JVZoo) don’t provide the best visuals – so I’ll just show you my total affiliate income for the past year. In one year (March 13, 2015 to March 13, 2016) I earned $3867.60 in commissions for promoting Long Tail Pro. Not bad for having just a mildly popular ‘passive income blog’ and only a small email list!
In a nutshell, Long Tail Pro helps you quickly find keywords in bulk based on a seed keyword that you input.  In addition to returning hundreds of related keywords, the tool also shows search volume (monthly search volume), advertiser bid, number of words, rank value, and my favorite Keyword Competitiveness (helps you judge the competition and keyword difficulty).