If you do accept an earnout, know what you're getting yourself into. If it's not on paper, promises mean nothing. Understand that you are selling your company, and giving up most if not all control over some very basic elements. Budget, hiring, partnerships, customer relations. These all change once you become part of another company. With so much out of your control, how can you expect to be responsible to hit the projections?

The best way to protect yourself, is getting a guarantee, in writing, that no one will interfere with the running of your group. Something will always come up, either a customer you have who is now a competitor, or you're interfering with the operations of another group. You will get a call from someone else in your parent organization. Don't argue, just ask for a letter to explain to the customer what's going on.

Once you have proof that you weren't able to run your division the way you wanted, go back to the original negotiators and request for the earnout in full. They won't give it to you, but you'll get something. Usually more than what you would if you didn't ask for it. Plus you'll get time, time to relax, and plan for the next company you'll start.