Just as there's a yarn for every occasion, there's a KNOT for every occasion. The Fisherman's Knot is quite possibly the best one I know for keeping two ends of yarn together - and it does so without creating a huge, wonky protrusion in your fabric.
by HappyBerry Knitting
by WoolyWondersCrochet
by Mad Stuff With Rob
by Kids Rainbow Art
by AdtCreativity