I prefer a headset-type microphone, which allows for some head movement while keeping the microphone at the correct distance from the mouth. Outside noise is usually not a problem with most voice recognition programs. If you are interrupted while using the program or have to respond to something going on around you, simply switch the microphone off using your computer mouse. This will prevent unwanted conversation from showing up in your dictation. Speak distinctly, not slowly.
Speed is much less critical than clarity when using voice recognition programs.
Both IBM's ViaVoice and Dragon NaturallySpeaking can keep up with a normal pace of speech. In fact, it is best to speak naturally rather than to intentionally pause between words. Dragon NaturallySpeaking uses context-sensitive algorithms in recognizing continuous speech, so speaking naturally actually improves its accuracy. Complete the training session. Follow the directions, and do not try to shortcut this training process, or accuracy will suffer.