I agree with virtually all of MissCreant's advice, except for one bit (ha ha): Speaking in low tones is not the best advice! I agree that a squeaky, annoying voice is not advisable for communicating with a frightened dog either, but be aware that low tones sound like a growl to a dog. In general, most mammals associate low tones with aggression, and higher pitches with happiness. Men, especially, must watch their voice pitches when speaking near an unfamiliar dog. Women have a better time training dogs due to their higher pitched voice.

If you're training your dog, low tones can be useful. Once the dog accepts you as its alpha, a stern low-toned NO! can be a powerful deterrent against unwanted behavior. If you are not recognized by the dog as dominant, however (i.e. the dog isn't yours), I would not recommend doing this. Be friendly with dogs you don't know. Avoid low tones.