Here's a quick tip...
If you have an established site with different parts then you can find keywords for that specific part of the site using this trick...
I've used it on Niche Laboratory to find keywords that the new Forum is ranking for. That way I can post some forum discussions relating to all these new keywords, and thereby grow the forum.
What you need to do is go into the Google Webmaster Tools Search Console and click on the Pages radio button on the page that shows all the search queries your site is ranking for.
You can then Filter your site's pages. In my case I filtered on URLs containing /Forum/.
This works particularly well if your site is already well established and you have more than 500+ queries you're already ranking for.
Oh, and you can also filter on specific pages. This is really useful if you want to add some more content to an existing page.
Hope you found that tip useful!
Want to find long tail keywords? This awesome keyword tool will find phrases like how does buy to rent scheme work and how to build a small niche website.