Think about your professional and academic background, and what it is from each that makes you suitable for the area you want to enter. Perhaps you completed an undergraduate degree in an area that is key to the practice? Review the specialized certifications you may have earned in law school, the subject matter of the journal you wrote on, and the relevance of your clerkships and externships to the area you want to enter. If you had a career before the law, consider whether the work you did prior to law school is pertinent to the work you want to do now. And, of course, analyze the areas in which you work now. Think about the substantive work that you do, how you interact with and understand other practice areas, the client industries you service, and the amount of experience you may have in subspecialties or in practices that area complementary to yours.
In addition to your past, think about what you can do to work towards the area in which you would like to practice. Consider charting your own education in the area by planning out a curriculum of CLE courses. This is a good idea not only for educating you in the foundation of the practice subject but, also for informing you of the current trends and issues in the area. If possible, seek out pro bono matters in the area you would like to enter. This is a good idea for demonstrating commitment to the new practice as well as for evidencing your ability to work in the area and with little or no supervision.
If you have a background that reasonably links you with a practice, or if you are willing to work towards justifying a strong enough connection to a new area, then you may have a case to make. Just remember that you will still have to work against a market that is resistant to change and that, in this instance, the burden of proof is on you.