Let's start with what you know.
Do you have years of law experience? Do you have an in-depth knowledge of the law you practice? Have you had many trials to hone your skills? Did you graduate with honors? What about your clients? Are they satisfied enough to recommend you?
None of that really matters. In order to be an expert you need media exposure and a lot of it. You have probably had this happen to you or you will. You're watching the evening news and some lawyer is introduced along with his credentials to establish credibility. After you get over your disbelief, you realize you could run rings around him in negotiations or trial.
So, why is he on the news or in the newspaper and you're not? Exposure and promotion. Sure, it costs 50 cents at Rotary if your name is in the paper...but it's worth it!
The advantage of exposure is familiarity and credibility. How do you get exposed? You schmooze reporters and return phone calls. If a reporter is on deadline (and they usually are) they are not going to wait around for you to call them back. They will go to the next name on the list.
Another way to get exposure and introductions is to hire a public relations firm but that is usually reserved for high-profile cases. Again, you can spend years establishing yourself as the go-to guy on an issue, or you can bypass those years and hire someone to do it for you.
What are the advantaged of being an expert and getting exposure. New business and prestige. Whether we want to admit it in polite company, money and recognition are the goals. Sure, we believe in the Rule of Law and respect it, but doesn't it make you feel great to be recognized as an expert. After all, we are only human and respect is a fundamental need.
Most importantly, becoming an expert means we can impart information to others. Law is law, but technique is everything. People look to you for guidance and advice. They learn from you. That is the quintessential quality of expertise.
One warning: It takes all of the above to become recognized as an expert. It also takes time. Not decades but years, so plan ahead. Do not expect to become a success overnight or hire a PR firm and become an instant star...although it doesn't hurt! That is, unless you have a few decades to spend working on it.