It's not too difficult, really. Widen your eyes and knit your eyebrows just so, to give the appearance of being a bit sad and helpless. Throw in a few blinks here and there to enhance the effect. Perk up your floppy ears. Repeat. If you don't get a puppy treat within a few minutes, give your person a few puppy licks. That gets them everytime. ;)

A wonderful how-to-guide on being cute by bartendermagic_com's stunningly cute puppy, who you can get more of on his photostream on Flickr.