"Display your makeup stash in such a way that you can easily see all of them at any given time. This way you'll be aware of the things you already have, and can avoid buying similar things. For me, lip products are my downfall. So I took a clear plastic box, put them inside the box and group them by colour/shade family. This way I can stop myself when I'm tempted to buy another fuchsia/berry/nude lipstick because I remember I already own like, a dozen of them."
More tips here.