What to do if your biggest client is a risk to your solo business

Every year, I look at how much money I earn from each client. Some years, my earnings are very skewed toward one client (or two). If something had happened to that working relationship, my year could have looked much different. Before I became a solopreneur, I worked in banking and then at a tech company. We had a phrase for reliance on a few clients: concentration risk. Companies that have this type of risk have a whole system for tracking it, and often try to find ways to reduce the risk...