In order to have a successful niche website you need quality content. I won't get right into everything about quality content here, as that's a tutorial in itself, but you need to be able to come up with content ideas regularly and write and publish consistently.

Think about your niche ideas and scrap any that you feel you won't have enough to write about, or won't be able to maintain enough enthusiasm to write about regularly. Even though you want your niche to be defined, you don't want to define it so much that your content ideas become extremely limited , and you find yourself basically write similar content for every post.

It's also a good idea to choose a niche that can be expanded upon later if you decide to branch out after establishing your site. This will often come down to the name of your website and your domain name. Don't choose something that locks you into an idea you can't expand on later. As an example, if you called your site or domain "Grooming Tips for Men", then you are locked into grooming tips for men only. It'll then be difficult to later start writing about grooming tips for women.