If you want to save your animation, select the 'Save & New' tab and click 'Save as active project'. When you want to start working on it again, find it in the "My Active Projects" section of your My Animations page, and click 'Edit'.
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Before you start on the beautiful details of your animation, try drawing a simple outline. This way, you can get the overall composition looking great, and you can make sure everything's going to fit in the frame! Once you're filling in the details, you can start deleting your outline if you don't need it any more.