1. Write an Article
Writing an article is an excellent option because oftentimes whoever you write for will inherently label you as an expert. You may want to start off small by writing for less popular websites/blogs, magazines, etc. Once you have some writing credentials you can then approach larger media sources.
2. Create a Website and/or Blog
This is a great and easy way to build your credentials. By consistently offering good tips and advice to others interested and looking for information on your given topic, you will begin to build a reputation as an esteemed authority.
3. Create Podcasts and/or YouTube Videos
Before you begin making your own videos, read "Top 7 YouTube Sins" so you don't make these classic blunders.
4. Offer to Speak at a Local College
This is best if you attended that college or university yourself, but if you did not attend your local college don't fret. If you present yourself in a professional manner and offer to do it for free, most universities are more than happy to have guest speakers.
5. Host Your Own Seminar
This is a great opportunity to get your name out and also make a little money. Get other speakers and local businesses involved as well if possible. The more people and/or businesses involved the greater the buzz you can generate.
Before you try to pass yourself off as an expert, you must first gain specialized knowledge. No one's going to believe you are an expert if you have no idea what you are talking about. Gain legitimite experience as well as gain as much knowledge as you possibly can. It's not hard to fake your expertise, but it's even easier to be caught as a fraud.