How about clear thick-ish glass? I use one for tea (two cups a day) and no stains! Ikea carries them. Bodum has nice double-walled coffee mugs if you are feeling more fancy!

Hard water can be a culprit. I noticed less staining of coffee and tea implements when I switched to filtered water.

A little bit of bleach will remove the stain easily. Just wash the cup,well afterwards.

I've had good luck with pottery ones. With some looking, you can find ones that look "more delicate" rather than "more homey", if that's your style preference. Just make sure it is made with a food-safe glaze.

Of course, you could always go for black! ;-)

Soak them in denture tablets !