DO Hire experienced, classically trained character actors for most of the regulars and not just for the ridgeheads. Character actors with no fame behind them are cheaper, more versatile, and better trained. I don't care about looks and neither should you. If you must have a Hunk and a Babe on the cast then do so, but please ask members of the opposite sex before casting them. Just because a guy is young and handsome doesn't mean he's sexy to women.
Many male actors are gay, and that's fine, but they have trouble projecting masculinity on-screen.
DO Make your sets and lighting conform to the atmosphere you wish to create. X Files always has grayish light which makes things spooky and foreboding. DON'T ever ever again put a regular character in a catsuit. No one believed Kira in a catsuit, which was so different from other Bajoran uniforms. The fans are laughing at you.