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 WoolyWondersCrochet
by crochet patternss
by MrUsegoodART
by Origami for Kids
by ArtParadise 1214
by Nagu's Handwork
by Noreva Projects