Making each character the hero of their own story. This is the approach I take for creating complex side characters, and I love the results, but it's a lot of work.
Refuel and recharge regularly. And I don't just mean food and sleep, although those go with it. But love your family, hang out with your friends, talk with strangers, read good (fiction AND nonfiction) books, learn a new thing... Basically, keep that creative well filled!
"Trust your instincts." There's a lot of noise out there about the industry and what every author 'should' do, but filter out the things that run contrary to personal conviction, no matter how 'shiny' the thing may be.
