If you don't already have the habit, I really recommend keeping a sketchbook or ideas book. It will pay you back many times over any time you spend on it.
Sketchbooks get you in an artistic frame of mind. They keep you practising which keeps you coordinated and used to seeing detail. They can also be such a relaxing thing to do. I love sitting and drawing in the garden.
You can also take notes as reference to use later:
or to teach yourself something you find difficult. I spent a lot of time in Europe drawing statues' knees and taking notes!
You can also use them to collect things you find inspiring - photos, postcards, pictures of other people's works, magazine cuttings etc. Then you have a resource book of ideas and reference material for other works.
Sometimes they can provide providence (a sort of history of inspiration and development) for potential buyers and gallery owners. People are often fascinated by artists' sketchbook, and some galleries even have exhibitions of just sketchbooks. I love seeing the sketchbooks of my favourite artists - seeing how they work through their ideas.
One of the best uses, I think, though, is as a souvenir. They can make invaluable mementos of holidays, or children growing up, of artistic phases in your life, of the change of seasons in your garden. If you haven't been keeping one, try and find a little time to start one today.