Writing: Read ELEMENTS OF STYLE by Strunk and White. Write. Write. And write some more. Remember, sometimes less is more.
"Brevity is the soul of wit."
Memory: I forgot what I was going to write :)
I can't really advise one sure way that has helped me out. Because what works for me may not work for you. What I can tell you is that you need to find and use what works for you. Some people use flash cards. Some people record themselves. I know that to remember something I want to say or do, I have to write it down somewhere in some form or I'm going to forget and it will not make me very happy. I hope that helps.