1. It must not be too tight (left) or too loose (right).  A proper fit (middle) will be just roomy enough to wear a hoodie underneath and be able to still zip it up.  You should NOT be able to fit small children, decorative pillows, or weapons [larger than handguns] inside your jacket.
2. Avoid lighter-weight, thin leather jackets....  they tend to appear like black garbage bags.
3. Leather jackets with elastic waists are NO-NOs!  It looks like you are a 15 year old boy wearing your dad's leather jacket from the early 90s.
4. Get a jacket with structured shoulders!  The sloping shoulders of the jacket (right) is really unflattering.  It totally detracts from the independent, tough-guy look.  Tip: The more seams it has, the more structure it has.
5. Stay away from floppy collars-- NEVER sexy.