In Auslan, the convention is to not mouth or speak the English words and sentences as you are signing, although some Deaf people do this to an extent anyway. For practise, try keeping your mouth closed as you sign. Avoid signing Auslan and verbally speaking English at the same time, as the English will confuse your Auslan grammar, facial expressions, depicting signs etc.
Auslan does, however, have its own lip patterns. These are the mouth shapes Deaf people make when they sign particular words. See the video for the lip patterns for these common words and phrases:
• Never seen it / never heard of it