You could talk in gibberish to your dog and he might give you a silly look, wagging his tail the entire time. Put a little oomph and deepness behind that gibberish and your little guy's composure will completely change. While what you say to your pup can be important once you teach him what certain words mean, the way in which you speak is crucial.
Maintain a non-threatening posture. The way in which you stand and walk says more than words could ever hope to. You want to appear confident and never aggressive or timid. A steady, upright posture tells your pup that you've got this under control. Leaning in toward your dog or hovering over him is one of the worst ways to stand, because you appear aggressive. When you walk, maintain a steady pace, never fast, and don't intentionally make yourself look larger than you are. Keep arm movements to a minimum, using them primarily for hand signals. These instructions might seem like you're in some kind of boot camp, but following them will help make your pup relaxed and ready to listen to whatever you have to say.
Speak slowly and clearly. While you don't need to slow down your speech to a few words a minute, your pup will understand you much better if you make an effort to sound out each word and slow down your speech just a tad. This is especially true if you tend to run words together.
Talk in an authoritative voice for obedience training and a more uplifting voice when counter conditioning or when praising. When telling your pup to sit, you want him to know it's time to be serious, at least for a few seconds. A higher-pitched, more uplifting tone often excites dogs, which can throw off their concentration. There is an exception to the rule: if you're trying to persuade your dog to do something, as is the case with the recall command, a more excited voice works wonders.
Pair words with actions. Without associating a word with an action, you might as well be talking to your pup in another language. For example, take the sit command. If you haven't trained him to sit, he'll just look at you and blink a few times. But if you say "sit" as he's about to plop his butt on the ground, and you reward him when he sits, you're pairing the word with the action. Just remember: say the intended command as he's about to do the action, and then reward. Your little guy will learn a lot faster that way.
Use one- or two-word commands. Oftentimes, the best way to communicate something to someone is in as few words as possible. The same holds true when talking to your pup, except on the extreme side of things. Your pup will have a very hard time understanding what you mean if you say, "I need you to sit," rather than "sit."
Speak with sharpness rather than loudness when your pup does something he's not supposed to. Bellowing out a deep "no" will grab your pup's attention, but it might cause his heart to jump into his throat. When your little guy gets in the trash or sticks his head into the kitty litter box, you don't want to frighten him. You want to slightly startle him, grab his attention and get him out of there. It's better to let out a sharp "ah."