According to Dr. Nicholas Dodman, use short commands such as sit, down, leave it, quiet, out, and off. Say the word once. Do not repeat the command. Dogs will remember a command for about two minutes before the notion is lost. Shorter words are better than longer words and words that end in a hard consonant (C, K, T, X) are better than those that end in a vowel because you can "spit" them out.
The only command that should have three sounds associated with it is COME. In this case, you first have to attract the dog's attention by saying his name, ROVER, then COME (the actual command word) and GOOD BOY, even before the dog comes so that he knows he is not in trouble. Make sure your tone is crisp and cheerful.
Dr. Dodman has some great tips in this article – please read 12 General Rules for Training Dogs