My first use of both Humbrol and MM enamels didn't result in stored remaining paint lasting very long, but I read you don't want to put thinned paint back in the bottle, (or tin.) Since then, any left over of thinned paint just gets ditched when spraying is finished.
Also I think it's really critical that the container be kept meticulously clean, around the threaded areas of both the bottle and the cap. For that reason I never shake the container to mix the ingredients, I first stir completely with a stick to loosen all of the really thick solids at the bottom, then use the Badger or other little electric mixer to get a fully blended paint.
I don't pour the paint over the bottle edge, I draw it out with pipettes, that way the threaded surfaces remain clean. Your idea of kitchen plastic wrap is a good one, I do that as well, can't guarantee that it works, but it sure can't hurt.
Now, by being careful with the bottle handling, I find I can store most any enamel for several years. Rarely do I find it gone bad due to drying up.
Humbrol seems to last a very long time for me as well.
Patrick