Adding sodium citrate to a cheese sauce is the best way I've found of making it less fussy. It's literally like pixie dust - you sprinkle some into some water,

Adding sodium citrate to a cheese sauce is the best way I've found of making it less fussy. It's literally like pixie dust - you sprinkle some into some water, melt some cheese in the water, and bam: smooth, creamy cheese sauce with no chance of it separating.