Like MamaB and Aldougie, I have found the straw cups the best way to encourage my daughter to drink (am using the Playtex and also Munchkin brand - from Wing-On or Eugene Club). I'm using diluted apple juice (about 5parts water to 1 juice) and she is lapping it up.
My elder daughter went solely onto formula at 9 months and for the next few years we had dreadful problems with her constipation. I know now she was lactose intolerant. After only a few weeks on formula she ended up with a tear in her bottom, and that was the start of her problems. I tried everything, prune juice in water, apple juice etc etc, but already she had had the painful experience and had started to withhold, which made her poos harder.
This time around, as much as I think water is the best thing they should drink (after breastmilk and formula), I want to ensure my baby has an adequate fluid intake, so I'm doing the apple juice and water combo.
Also, I've discovered that the prune and apple (or is it prune and banana) Organix baby porridge is excellent for keeping babies regular. Unfortunately it seems to be out of stock. My baby would wolf down a bowl of that every breakfast.