A youtube channel by Ann Yorks of the Flour Box Shop that is dedicated to cookie decorating. You'll find lots of useful tips about techniques as well as fun ideas for cookie decorating.
Normally, when you bake a cake, the centre of the cake rises higher than the edges do. Using a simple technique, you can get a flatter cake that is much easier to decorate.
Serdar Yener's youtube channel that is all about teaching people how to make beautiful and interesting cakes. He shares many tricks he's learned along the way that will make decorating your cakes much easier.