I would say always take your time and do a muslin first in fabric as close as possible to the final fabric you will use. And don't sew when you are tired; you are more prone to make silly mistakes. Also it is important to always PRESS as you sew to get a professional result.
I would love techniques on how to work with fabrics such as silk chiffon; specifically how to create a professional finish. I think sometimes it's a case of 'you don't know what you don't know' and any tips would be great.