I have a necklace with an adjustable knot and was wondering how I could do the same to my other necklaces. I Googled it today and found this easy to follow tutorial. You can find it here: http://www.instructables.com/id/How-to-tie-sliding-adjustable-knots-for-Maori-Neck/
I just tried it out, and made these knots for four of my necklaces. Now I don't have to worry that the string is too long.