Sometimes we think about this and try to bring our own mug to the coffee store, but someone can say it's inconvenient, or it's big, or we forget, and so on. But there's something else we can do: take the sleeve out of the problem. It's not everything but it could be our little contribution. And most of the times, it's better to do something small than nothing at all. Doing some quick online research I found that cup sleeves make 2.8 billion pounds of waste each year; that sounded way too much trash that we can easily avoid by reusing the sleeve.
I think a good way to do anything you have to do is to make it fun. That's why I'm making these cute coffee cup sleeves that you can carry in your purse, leave it in your car, have it at work, slide it in between paper stacks, etc. They are all hand made with 100% cotton and have a Velcro closure, so they fit easily in 12oz, 16oz and 20oz cups. Some are made of printed canvas and some others are made from a thinner cotton and have also a layer of an insulate material so you don't burn your hand (the canvas' ones don't need it).
Email me if you'd like to help the cause by using one of these, or if you'd like to give it to someone special and spread the word.