If there's nothing else you take away from this video, know this: Dogs, and especially puppies, thrive on consistency. If you want them to do a down command, ALWAYS say "down" - don't ever switch the wording.
If you want to communicate more effectively with your dog as well as train him to obey, check out this video for tips.
Chances are, when you're cooing to Fido, all he's hearing is, "Blah, blah, blah." Stop annoying everyone around you—and learn to speak his language.