There's alot...ALOT of schools of thought on this, but here's what I've learned:

You can either lean your body with the bike, lean the bike more than your body or lean your body more than your bike. Here's the ideal situations for each --

FAST, sticky turns - lean your body with the bike. This seems to help carry momentum and if you're going fast enough, you will really just do it naturally.

FAST, but loose turns - lean your bike more than your body. This helps to plant your weight more on the tires. Of course, you ALWAYS try to plant the outside pedal at 6 o'clock.

LOOSE, really loose turns - lean your body more than your bike. You may not even hardly lean the bike. This tends to help put momentum in the right spot, but keeps your tires at the maximum contact position/patch.

Now all this gets more complicated when you throw in roots, mud or improper braking.

BTW, braking and therefore setting up for the corner is just as important as going through it - most people say brake before you ever enter, some like to trail-brake or just kinda ease off as you pass through the corner. If you are really comfortable with your bike and the terrain, trail braking can be faster. But on those days with adverse conditons, getting tuned in before you go in will yield some faster moves.