Need to be really specific about your niche if you're into health area. Broad keywords are saturated. Instead of 'diabetes' go for something like 'Best Diet Choices for Diabetes'. Market will be smaller but more concentrated and hence more likely to boost affiliate earnings.
Passion and Ever Green niches are good. Take a half a year to learn something and become an expert. Join other blogs, forums, and Fanpages that are specific to that niche. Engage with others and ask questions. Then go back to your blog and write a 500 article about what you just learned. Be sure to ask your readers questions in your article. Let them engage you in the comment section.