Good old youtube you have to love it, The "Recommended for You" section is very handy it knows what videos you like and suggests things.

Most recently it suggested this video to me.
http://www.youtube.com/watch?v=tQxdCfbE2FI

In the video he manually holds the ND filter and feathers it to smooth the edge of the filter sounds like a good tip to me he was even managing to do it for a 6 second image.

Think I will have to give this a try