We started offering our DS water at about 3 mos, during a hot spell when we didn't have a/c. At first, we just offered a little bit (1-2oz) in a bottle. We've since moved sippy cups and he'll drink water out of pretty much anything (even my water bottle!) if he's thirsty. He seems to like the Nuby valve-less soft spout best. It's skinnier in the middle, and he's able to hold it himself, sometimes.
We usually offer water 2-3 times during solid feeds, and also if we're outside much during the day (warm weather and no a/c in the car). He seems to really like water. Our doc wasn't too crazy about it -- "formula ONLY" -- but agreed that as long as he doesn't get more than 2-3 oz/day things should be okay.