How can I improve "morphemes?"

[amandaraemay]

Hello, I am hearing. I can't really say that I'm a "student," but I have taken ASL classes in the past. A very good friend of mine is Deaf and we spend a lot of time together, so my signing improves daily. I am always looking for ways to improve my signing by asking for advice, and constructive criticism. Others have said that a way that I can better my signing is by working on my morphemes, or how I use expression in my signing. I feel that I can express emotion quite well, but what are some general 'rules or guidelines' that I should or shouldn't do? I sometimes fall victim to signing in more of an English or PSE order, opposed to ASL. Is it inappropriate to mouth words when signing? Or to do so when emphasizing? Any suggestions? Thanks!