These ideas have helped me too as my DS doesn't drink a lot either - so thanks!

One thing I've come up with is breaking up an ice block (the 99% fruit ones) into his sippy cup with water in it. It flavours the water and he shakes it around to see the ice melt. I think he also likes it colder (like I do!). My big craving when pregnant with him was ice and cold things - so I figure he'll like cold drinks!

So maybe the temperature is something to play around with too? Some kids would prefer cold, some room temp...