The best tip is keep the heat low. Cheese needs enough heat to melt and blend with other ingredients, but if heated too long and at too high a temperature, the cheese can become tough and stringy, and the fat can separate. A process cheese will melt evenly and smoothly with no fat separation. Process cheese can be cubed to speed melting. When cheese is used to top an entree, add it towards the end of the baking time so it has just enough time to melt.
Cold cheese is easier to shred and slice.