You purchase a large ($ millions) of umbrella insurance coverage.
most if not all personal umbrella insurance will not cover professional liability ... you may try to purchase a catestrophic (professional umbrella) but they are rare/expensive
Usually when judgement is larger than insurance coverage, even after the verdict, your attorney can still discuss settlement issues with the plaintiff and come to an agreed term that is lower than the judgement (and hopefully within your insurance limit)
Why would the plaintiff agree to settle post-verdict for a smaller amount that was won in court? Because 1. the plaintiff will get the money right away, instead of waiting for months/years as the appeal process proceed and 2. you can declare bankruptcy, which means any judgement that the courts attaches will have to go before a bankruptcy judge along with other creditors (and the plaintiff may get a fraction, if any at all).
Some states have a mutual catastrophic fund (that all physicians pay into) that pays out when the limit of their professional liability insurance hits its limit.
Also, plaintiffs will likely go after the hospital and health system since they typically have more money than the physician and his insurance policy. Most states operate under the joint and several liability concept so that regardless of your role in the malpractice (even if it is 10% as determined by a jury), if the hospital can't pay up, then you are responsible to the plaintiff for the entire amount (and not just 10% of the amount) - although usually it;s the other way round where physicians can't pay up so the hospital is on the hook (it doesn't relieve you of any future obligations though, since the hospital can go after you for your share of the award that you didn't pay)
There are various ways that you can shield some of your assets from the courts - but you really need to talk to a financial planner and lawyers since it will depend on local laws as well as federal laws. They may involve setting up trusts, or creating a business entity (with you as a sole employee), etc. But they can get very complicated ... I would recommend talking to a financial advisor as well as a lawyer that specializes in asset-management and protection. Be aware that there are shady advisors who will try to sell illegal tax shelters and illegally hide assets - not only does it not protect you, but exposes you to additional risk such as criminal charges, etc. Plus you don't want to deal with the IRS
This isn't just limited to medical malpractice. You are similarly liability for your personal activities as well, even if the damages exceed your assets. If you were to cause a multi-vehicle accidents (expensive cars, trauma patients, etc) your liability can easily exceed your policy limit. If not properly insured, your assets are at risk. And lawyers are more willing to go after a successful doctor (with bank accounts, cars, vacation houses, etc) than a poor convenient store clerk who only has $150 in his bank account. (they can also garnish future wages to satisfy judgements)