I recommend a bit of research before you start spending money on various tactics. Ask any doctor friends or current clients what would stand out to them, what would make an impression and what they recommend for your new practice. I've seen firms even set up short interviews with office managers at local practices to simply "ask a few questions to get your advice as I expand my tax practice to the medical field." You will get great advice while at the same time getting an opportunity to let them know about your practice too. Hear what they say, and make note of it.
As far as direct mail specifically, it may work better than you think if it's unique and fun. Something that shands out will get passed around the office sheerly because it's an interesting piece of mail. You may also want to see about writing (or having ghostwritten) an article or two for the local medial publication or see if you can get a spot to speak before a local doctor's association. Another tactic that I've seen work well is to start a blog with short posts and real practical tips for those in the medical profession. While doctors are somewhat competitive, the targeted nature of their practices mean they are usually happy to pass information and referrals to their colleagues, so make it easy for them to do so. If they read a blog post about a new tax law or strategy that may save them money, they will be likely to pass it along.
Finally, make sure your Website is written in such a way that it is client-focused and talks about this practice area. Medical professionals, some at least, like to be the center of attention, so if it looks like your medial practice is small and a new "add-on," they will be less likely to call. Put them front and center, at least on your services page, and include a link to that page in all your correspondence.
Feel free to contact me with any other questions. I'd be happy to help. Cheers!