First step of On Page SEO
Google keyword only gives you suggestions which are directly related to your original keyword, leaving out the ones which are close to your niche, but not direct ones. These keywords, usually long tail, may be very profitable and have less competition.
Think about your main niche and jot down other similar niches. Form Niche clouds. You can do this using mind map, either software or just pen and paper. If you get stuck, search forums. Many forums have sections which can be additional niches.
Wikipedia articles are a great source as well, check the table of content for your main Niche.
The original, full article can be found here, includes step by step instructions.