Planting a foot when cornering: when and how?
While a "dab" isn't exactly the most graceful of techniques it's useful to save yourself from a spill or wipeout. What you need to ask yourself though is whether you leave a foot out because you're not sure what's going to happen when you turn or because you plan on going fast enough to slide around the turn. Sometimes the slide is unexpected and should just quick plant a foot and move on.

On the other hand, you're going to get a bunch of trail builders and riders who say that any slide is unwarranted and damages the trail. Generally this is true, but hardpack trails with dust on top or a little gravel are very easy to slide on and don't damage the trail at all. Mud, loam, and other loose soils are more prone to being disturbed by excessive sliding.

As for the proper technique for a dab, it's very dependent on your riding style. Here's a general breakdown of the timeline for a turn:


See the turn: Look ahead and see a tight/loose corner. Begin evaluating how you want to take it (eg. at speed, slow and upright, or sliding).
Begin the turn: Lean the bike towards the inside of the turn, pushing your weight to the outside pedal. Keep your inside foot relatively light.
Back tire breaks loose: Knowing when this will happen requires experience with your tires, so practice sliding in a parking lot or gravel road.
Plant foot: Remove your inside foot and place it slightly ahead of where you currently are, in the direction your moving.
Push through the turn: Push a bit with the inside foot, kind of a cross between a kick and step. This should keep your bike position or possibly move you a little more upright.
Re-engage and continue: Plant your foot back on the pedal and pedal out of the corner.


And that's more-or-less how it's done. Watch a bunch of Trail/AM/DH riders do it and you'll get a sense of where in the turn you should expect a plant.

Last bit, as you get more confident riding push more speed into the corner. You'll find that some spots where you previously needed to dab, you'll clear no problem due to the added force keeping your tires in place.