New MTB Best Practices

Hey everyone,

We've been getting a ton of "which bike is best for me threads" (which is great), but rather than search my comment history to copy and paste my last rambling piece of advice, I'd like to create a "New MTB Best Practices" thread to keep linked in the right navigation pane.

Of course we'll still get (and answer) "which bike" submissions, but I thought it'd be nice to have this thread as a cheat sheet of sorts.

So...what's your generic advice for someone purchasing their first MTB?