Based on this thread I visited my LBS to see about some insoles as I was getting the painful pressure on the ball of my foot/hot foot after about 40 miles.

I have extremely flat arches - I'm talking damn near diamond shaped feet. But I ended up getting the specialized bg blue insoles. I was a bit skeptical, esp at $44 bucks that something replacing the now paper-thin insoles in my Mavic Avenirs would make much of a difference. I used both of the orange corrective varus shims under the insole because my knees rotate inward alot. Then I headed out on my normal tt loop (22mi).

The difference was immediately noticeable.... My foot doesn't move around in the shoe and I could put more power down without too much pressure on my knees and spin more efficently . Hey, these stupid shims work! Awesome! I went a bit (about 1mph) faster and suffered slightly less.

Am I saying an insole change is going to automagically make you faster? No. But if you have serious biomechanical issues in your feet and knees, the insoles may help you recover some of that lost power/efficiency.