Two things I do that are a bit uncommon:
I don't like being sole arbiter of rules as GM. When a rules question comes up in play, I like to look things up and walk through them with the whole group. That way, all of the group's rules experts can weigh in on matters of interpretation, and everyone learns how it's supposed to be done. We don't always get things exactly right the first time, but after a couple of walkthroughs, everyone is solid on how to do stuff, and things go smoother in the future.
I don't worry about trying to beat the players. A lot of folks believe strongly that a lack of challenge will bore players, which is true to some extent. However, constantly undermining their strengths, or poking at their weaknesses, will also cause frustration and boredom. It's important to threaten or challenge them sometimes, but it's equally important to let them do cool things. And when you do aim for challenge, don't just try to kill off PCs. There are lots of ways to fail in a mission, and there are Pyrrhic victories too.