Forgot to mention I change my needle after each project and it really helps. Years ago I didn't do this and although there wasn't a real problem when I started changing after each project I was amazed at how much better my machine sewed. Go to:
http://www.superiorthreads.com/education/
Bob will tell you what needle size to use for your particular thread. Even if you don't purchase there you will find a wealth of helpful information.