Before you begin, you need to make sure that the room you are recording in is as quiet as possible. You do not need an acoustically sound-proof room, but you need to make sure that, while you are recording, there are no external noises that your microphone might pick up and which may render you speech audio files unusable for Acoustic Model creation purposes. Use common sense: you should not have any music in the background, no fans, air conditioners, microwaves, television, etc. In addition, make sure you turn off you speakers while recording - to avoid acoustic feedback in your audio files.
This Discussion Thread provides more ideas.