As I usually do on these foot discussion thread, I plug the book "Fixing your Feet", usually available at REI or Amazon. Excellent ideas from an expert.

When you are losing toenails, your toes are hitting the front of your boots. The boots are either too small, or they are not laced tight enough. For long downhills, generally, one would lace differently than for other grades, to protect the feet.

I have yet to see a boot that comes with an insole that is worthwhile---they are all garbage! Look for an aftermarket insole, such as Superfeet, that will actually protect your feet.

I agree with Harvey that Benzoin is a secret weapon for feet. The full name is "Compound Tincture of Benzoin", which is NOT the same as "Tincture of Benzoin", which is not for skin, but a respiratory inhalant from years past. It forms a protective layer on the skin, and makes the skin VERY sticky to tape, so it will stay put. I use it a lot for foot repairs on the trail.

Additional references:

http://www.gorp.com/parks-guide/travel-ta-achieving-booted-bliss-sidwcmdev_053439.html

http://www.fieggen.com/shoelace/lacingmethods.htm

http://www.backpacker.com/gear/5245

I spent a couple of weeks a few years ago up at Vermillion Valley Resort, owned by a friend of mine, Jim. He is also an expert on foot and boot problems. The whole time we treated various JMT hikers who came in with horrible problems. Most could be made better, but not all. One lady had a blister consisting of the entire bottom of her foot, from her NEW boot. Yikes! Hurts to think of!