Gret question. This is a difficult technique. Don't 'glue' your palm to the strings when you do the downstroke-palm mute. The strumming arm/wrist movement is more important than the palm-mute itself. Don't sacrifice the fluid arm movement for the palm mute sound. As you are strumming, have your palm land on the strings at the same time your pick does. But don't keep it there! It's like a 'bounce,' you bounce your palm off the strings. That way, your hand is free to do the upstroke cleanly. Hope this helps!