How do you stop the white sauce from getting a skin when it's cooled? I make a huge white sauce for making up ready meal lasagnes for the freezer (I have teenage boys who eat at odd times) and it's all smooth and silky when it's hot, then by the time it's cool enough to start making up the lasagne portions, it's got a skin which turns into lumps when I stir it 
I've tried putting a lid over it while it cools, stirring regularly during cooling - no cigar!