These days, when I go to grab coffee, I bring my own mug. Always the same mug, a promotional one from Much Music USA. My friend Laurie gave it to me when she was working there and we were both living in New York, so I've kept this mug for at least seven years, across three different cities.
It's very lightweight, made out of some sort of plastic, and it never gets hot. It also has a plastic lid so it's safe to use it near my laptop and valuables. It's shatterproof, so I can throw it in my bag when it's empty and not worry about it breaking.
I probably get Blue Bottle coffee with it at least five times a week, whether it's at Cento by my office, in Hayes Valley by my apartment, or during my weekly trip to the Ferry Building farmers market. The baristas hand it back to me sometimes with questions about it or just a compliment, but always filled with a soy latte and topped off with a fancy design.
I've surely saved a tree or two with my reusable mug. Best mug ever.