Assuming you are taking care of all the other sides of your medical billing business it is imperative that you keep selling.  It's all too easy to fall into the day to day management of the business and is the quickest way to have a mediocre business.   

Selling includes direct mail, resume marketing, internet, cold-calling, etc.  The medical billing business is like any other business and needs fuel (new customers) to keep running.  Once you gain your first couple of clients (this can be a significant challenge as physicians will typically want to see proof that you can do the job) and you do a good job you will find this business will thrive with referrals, which is the best type of marketing.

Physicians talk and when you are good, they will give you more business than you need, but it's getting that first couple of clients that may be difficult.

The best tip I can give is to never give up with your marketing efforts.  Consistency and repetition are key in keeping you name "out there".  Even if(when) it takes longer to land the first client or your business isn't growing as fast as you would like, believe in yourself and be passionate about the services you have to offer.  If you aren't passionate about what you do, you may likely become easily tempted to give up when the results you want don't materialize when you think they should.