We have given DS a sippy cup (with a hard spout, no valve, 2 ears) since the 4th month, because he resisted the bottle and DW was going back to work. He (tomorrow 9m) uses it very well since 6~7m (unaided)! I give him the cup after every meal or snack of solids to help clean the teeth and prevent constipation. He loves drinking just plain water (in fact, this is often a good way to prevent him from getting bored when sitting at the table)!
I think practice can make perfect.