We have a tomme tippe cup with a straw and its basically in her line of site for the day. She'll go up to it whenever she wants a drink. At meals times, I put it next to her and then pick it up and say, "do you want some?" If she says no, then I'll pop it back down but normally she'll then pick it up herself and take a drink.
We starting using the tum tum straw cup and thats how he started having more water otherwise other cups he refused it