Use a re-usable water bottle

I stopped buying beverages few years ago and started using tap water instead. At the beginning, I used a thin PET bottle, like the ones you can often buy water in. However, I noticed that the water quickly started tasting funny - sometimes in a matter of hours. So I switched to glass, because that's impermeable. It helped, but the glass bottle was a lot heavier, of course, and when you want to take it with you in a bag, you always have to be careful. Now I use this thick, reusable PET bottle that once contained Vanilla Coke. It combines both advantages: It's lightweight, AND it doesn't let any funny tastes spoil my water or tea.