I don't usually keep track, but what I've typically done in the past is count drinks. I like to keep water and coffee or tea or something nearby when I sit to do something and for every drink of "whatever" that I take, I take two drinks of water immediately after. That forces me to drink at least roughly twice the amount of water as anything else that I drink. Otherwise I keep a pint glass on top of my fridge and randomly through the day I will fill it up and gulp that down.