How To Avoid Trapping Yourself in a Box
A niche isn't meant to be a trap but unfortunately for some people that's exactly what it becomes because they limit themselves too severely. It's amazing to be able to be specific about the types of clients your marketing to, and the area of law that you're looking to practice. It's less amazing to restrict that list to the point that you end up with a client pool far too limited to ever replenish itself.
As an example of this, if you're an environmental lawyer it's okay to limit your client base to people who've been affected by corporate negligence in public spaces. It's even okay to limit that list even further so that you'll only represent individuals who may otherwise have a problem getting a lawyer because they don't have the ability to pay upfront. But when you continue to narrow that list so that you're only interested in clients of a certain age and gender you then create a situation where your market may be too narrow to sustain itself.
It's important that you have a clear focus, just make sure that growth can comfortably fit into that vision.