Pick a business name for your practice
Note that your legal business name doesn't have to be related whatsoever to your marketing name! Choose a business name quickly and don't drag your feet. You want to incorporate as fast as possible so you can start engaging witih vendors as a legal entity.
Pick a marketing name for your practice
If you're very well known in the community, you may want to include your name in the practice name for marketing reasons. If you plan to hire additional physicians or take on partners at some point, you probably don't. Click around on the DPC Frontier Mapper for ideas.
Consider who to accept as patients
Be decisive! Going back-and-forth on your practice name hold up many other aspects of planning your practice: incorporation, marketing, contract
Decide on your business hours
Consider whether to have posted business hours at all. - if you only intend to be available on request, you may not want to commit to a pre-specified workday (though if you intend to have front-office staff, you probably should for their sake).
Decide what services to offer
Not everything has to be 100% covered by a membership (though of course, you should include as much as is feasible). Many practices offer crtain additional services—procedures, OB, physicals, prevantative health screens, etc—on a cash-pay fee-for-service basis. Consider what to include in the membership, what to provide at-cost, and what to charge an addition fee for.
As a DPC doctor, you're the linchpin of your patients' care. Don't be afraid to expand your scope of care! There are DPC practices that do joint injections, lesion removal/biopsies, cryotherapy, vasectomies, hemorrhoid excision, casting, cosmetic procedures, abscess I&D, OMT, immigration/DOT physicals, stress/VO2 max testing, migraine treatments, bone density testing, body composition analysis, travel medicine, CPAP, even coordination of hospital care and obstetrics. Watch this excellent talk by Drs. Lassey and Tomsen on expanding your scope of practice.
Decide whether to give your practice a specialist "flavor"
Some DPC practices have a bent towards a particular specialty/population, including pediatrics, geriatrics, sports medicine, PT, addiction medicine, pain management, endocrinology, wellness/nutrition, and more. Consider doing something similar if you're more specialized.
Decide whether to run a "pure" or "hybrid" practice
There are many shades of "purity" among DPC practices.
Some hybrid practices still accept insurance from a subset of their patients, or for certain non-included procedures. This is common for practices that are transitioning from a traditional practice to DPC. This discussion is laid out in more detail by DPC Frontier here. Also note that billing any insurance company unquestionably makes you a “covered entity” under HIPAA. A pure DPC practice may not be.
It is technically possible to run a DPC practice AND bill Medicare for certain procedures, if the benefits of practice membership explicity include only "non-covered services" (listed here), though that is highly restrictive. See DPC Frontier's analysis here.
Also note that billing any insurance company unquestionably makes you a “covered entity” under HIPAA. A pure DPC practice may not be.
Choose a pricing structure
There are lots of options: flat rate, age tiers, couple/family plans, annual discounts, or some combination thereof. Some practices charge a one-time enrollment fee. Others have no initial fee but charge for re-enrollment. Some practices charge a low per-office visit (something that my be required depending on your state regulations).
Some practices also do access/quality tiering: for instance, a Premium plan that includes guaranteed same-day scheduling, more comprehensive preventative testing, home visits, increased personalization, etc. This is an effective way to do some price discrimimation: if there is a wealthy subpopulation in your area that's willing to pay extra for preventative care, let them. However, you should make it clear in your marketing materials that your members are paying for care, not access. Charging for access to care could lead to being classified as an insurance plan by a zealous insurance commissioner.
Click around the DPC Frontier mapper to see what other people are doing.
Decide whether to do in-house dispensing
In-house dispensing is a great perk to offer your patients: if saves them trips to the pharmacy and a lot of money besides. Some states also require licenses, others impose limitations, and other make it nearly impossible - see DPC Frontier's state-by-state legal analysis for description.
There may be additional administrative overheads associated with buying and tracking Rx inventory, but it doesn't have to be burdensome. You can purchase pre-packaged pharmaceuticals from wholesalers like AndaMeds or Henry Schein, and re-distribute them to patients in the original packaging. If you prefer more fine grained control, you may need to purchase a pill-counter and Rx inventory tracking software.
If you do in-office dispensing: decide whether to mark up medications
You can mark up meds quite a bit and still offer your patients a great deal compared to the pharmacy prices. It's a good source of additional revenue for your practice.
Decide whether to markup labs
Just as for medications, it is possible to mark up the cost of lab work while still offering your patients a great deal.
For pathology services, some states have laws against price markup by physicians; see the "anti-markup" states listed here. Still more states require disclosure to patients of the actual cost charged by the lab (see "disclosure" states here). You should probably be doing this anyway for the sake of price transparency.