Nick B - that's a dangerous answer. I almost choked, laughing.
Websclubs - we've always just let the child decide. Over time, they reach for big cups, a sign they're interested in big-boy or big-girl cups. When we noticed that behavior, we just bought sippy cups (lids/no-spill) and over time it just became more interesting to sip a big cup, especially with the laud and praise associated with it.
Bribery with praise seems to help. Having a negative consequence never did, though (not when it came to behaviors that weren't inherently "bad" at least).