It's all about keeping it clean. Wash daily before filling with hot, just boiled water and dish soap - shake it up then rinse. Scrub weekly with a bottle brush, dish soap and hot water to disrupt any biofilm that is trying to form. Make sure the water is above 140F when you put it in and cap right away - mycobacteria cannot survive this temp for 30 minutes.
Finally, every vessel you use for drinking should be washed & sanitized. I use the dishwasher, which heats the water to 145F, with the heated drying cycle for all of our dishes, glasses, etc. If hand washing, our tap water is 138F - it is important to use soap and a cloth or brush on each piece when handwashing too - not just a hot rinse. Don't forget the glass by your bed and the one in the bathroom - they are great breeding grounds for airborne bacteria if you let them sit moist for days on end without a good wash.