Make sure you cook your roux based sauce thoroughly.
This is where many people go wrong. The roux needs cooking on a low heat for at least 5 mins before you add the milk and once all the milk is added you need to cook it until it stops thickening and no longer tastes of flour, this can take a further 10 mins.
Lots of cooks (even professionals) lack the patience for this.
Don't forget the mustard and don't be shy when adding the salt. If your cheese is a bit too mild a good cheat is to add a few drops of Tabasco to give your sauce a bit of bite. Don't go mad, one or two drops is enough.