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!
For a long time, I wanted to develop a tool for content marketers that would assist in the process of coming up with blog ideas and subjects. Upon researching developers for this concept I had, I stumbled across Long Tail Pro, and realized instead of developing a tool myself for fellow bloggers and content marketers, I would throw my support behind this one.
If you're a regular blogger, these are probably the topics you blog about most frequently. Or perhaps they're the topics that come up the most in sales conversations. Put yourself in the shoes of your buyer personas -- what types of topics would your target audience search that you'd want your business to get found for? If you were a company like HubSpot, for example -- selling marketing software (which happens to have some awesome SEO tools ... but I digress ;-) -- you might have general topic buckets like "inbound marketing," "blogging," "email marketing," "lead generation," "SEO," "social media," "marketing analytics," and "marketing automation."