The Watercolorist's Essential Notebook by Gordon MacKenzie contains a lot of useful tips. Things I haven't come across in other books (and I've gone through many over the years), including how to hold your paint brush and how to twist it to vary the thickness of lines. Here are a few of the doodles I produced last weekend with leftover watercolours. I love these rusty colours. I'm not sure how to use these techniques in an actual painting, but they might come in handy for card designs!