When sewing for yourself I think the biggest thing that makes things look more professional is making sure you have the correct fit. So learning how to adjust the pattern for your individual proportions is (I think) essential.
For tips and techniques some of that will come down to whether you want to make clothes like Alabama Chanin or like Chanel.
I really like old school type books for learning techniques like the Singer Dressmaking course. My favorite site for finding them was vintagesewing.info but now it's gone

but you can still access most of it using an Internet archive like waybackmachine.
Googling couture techniques can help too
ETA - there are a number of pdf docs linked here
http://www.ca.uky.edu/hes/index.php?p=170 that have lots of different aspects of sewing covered.