Maintaining what Seth Riggs calls the "speech level" in your voice while singing is simply a matter of making sure the neck and tongue muscle groups are independent of vocal production. Here's a great exercise: try sticking your tongue out and down, exaggerating at first, while vocalizing the sound "aaaaa...." as in "hat" through your complete range. As you ascend, don't allow your tongue to pull back in; just concentrate on it sticking out. Once you've done it a few times, try the same with your head rocking back and forth as if you were saying "no"; now you've distracted tongue and neck muscles completely. Continue through your whole comfortable range, but always remember: with any vocal exercise, only sing in a range comfortable for you; be patient--your range will improve in a very short time.