Personally I think this whole early sippy cup thing is a waste of time in many cases. Our pediatrician, whom I highly respect, suggested starting early with a cup too, but honestly my little guy wasn't interested. When a child is ready they will grab that cup and go for it. Until then, you can spend a lot of time coaxing them (and that's great if you have the time and interest) or you can just wait until they start grabbing for your glass - then you know they are ready. A child desiring to do something will make it happen much faster than anything else. (Note - in the case of delayed or ill children, you may need to put in more "coaxing" time to help them.) My guy just dropped it, dribbled liquid all over and eventually figured out how to rip the tops off and fling it across the room for fun - so we stuck with bottles longer. Everyone was happy and I didn't spend all day cleaning up after him. When a child does show interest it will be a function of muscle development and hand/eye coordination to use it, both of which demand lots of practice, just like walking, so don't worry - just keep trying when YOU think your child is ready!
SAHM of seven, 23 yrs - 22 mos