For when you are called on to say the prayer out loud, whether in a small group or large, you must take the feelings of others than God and yourself into consideration. Now isn't the time to try to berate a brother or sister by praying for him or her in front of other people!
Good articulation and projection are important too. The Lord will know why your head isn't bowed as low as it could be if you're in an auditorium and the microphone isn't working.
My dad likes to tell the story of a church leader who bowed his head low and mumbled when he prayed. When people complained that they couldn't hear him, he would say, "I wasn't talking to you anyhow."
If you pray in a group, don't use King James English and big, fancy words to impress people. Especially if you don't normally talk that way and wind up using flowery words and phrases you can't understand yourself!