You can add some flour or cornstarch to the melted cheese to prevent the protein molecules from coming together & clumping. The starch in the flour will coat the fat & protein molecules of the melted cheese so it will keep the molecules from coming together & maintain the flowy nature. You can also add lemon juice or white wine to keep the melted cheese smooth . The acid will bind to the calcium molecules & prevent them from getting linked with the proteins. Thus, the protein molecules will be unable to clump together. The added acid will also help to contribute some liquidity to the cheese thus keeping it from hardening & becoming stringy.