Nuts?

Has anyone found a method of making bird feeders squirrel -proof? 

So far all efforts have failed.

The latest attempt to foil these :"little critters" was a special offer in a newspaper of a squirrel- proof feeder.  Unfortunately it is so impenetrable the birds can't get at it.
We have grey squirrels yet two hundred yards down the road a neighbour has seen red squirrels in her garden.