This is anbalagous to your personal will (which I discuss in detail on the Estate Planning Page). A professional will stipulates what happens to your professional possessions (rather than your personal ones). For most of you, that means your private practice. 

It's important to keep in mind that not just anyone can step in and mange your practice. In most cases, you'll need someone who holds similar licensure. This can get a bit complicated, so it can be a big help to work with an estate planning attorney who is familiar with planning for licensed professionals, or even specifically licensed mental health professionals.

That said, I'll repeat what I say about all elements of financial planning: even a small, partially thought out plan is infinitely better than no plan at all. 

That said, to get started you don't even need to work with an attorney at all, you can just write down what you'd like to have happen to your professional assets. Unless you're looking to legally transfer ownership of your practice's financial assets, your primary concern here is that your clients receive continuity of care. And as long as there is some type of plan in place (regardless of whether you use an attorney or not), your clients should be in pretty good shape.

I will say that just having your professional will filed away in a drawer somewhere usually isn't going to be enough. You really want to have an emergency response plan and team in place, which is what I'll cover next.

This is analogous to the organization and documentation you put in place in your personal life, it's likely just a bit more formal. And where you have a few close family members or friends that will likely step in and be your personal emergency response team, for your private practice and your clients, you'll probably want a more formal emergency response team.

And I really do like the idea of having a team instead of a single colleague. If you have a full caseload, suddenly and unexpectedly transitioning responsibility for all those individuals to a single person would be a real burden. Having a team provides a way to share the workload among a group.

Another great reason to have an emergency response team, is that the members of that group can help facilitate and hold one another accountable for putting together the emergency response planning in the first place. This type of planning is a lot of work. And it can be overwhelming. You will likely run into questions and uncertainties along the way. Having a small group to work through it all together can be super helpful.

Private Practice Preparedness. This book provides a comprehensive overview of how to establish a continuity plan for your private practice, and also includes customizable forms.

CAMFT Private Practice Corner Page. The California Association of Marriage and Family Therapists offers several articles in the "Will Writing & Office Preparedness" section.