For the knees, elbows and back: Always use a gentle scrub. After exfoliate, apply a very moisturizing cream because these two areas of your body are usually very dry and need more hydration, for the back you can assist it with long-handled brushes.
For the feet: Use a scrub mint and apply it on your wet feet. You can help yourself with a wooden brush with bristle special for this area.
For your legs: The brown sugar based exfoliating are ideals, they can be found in leading brands of products for body care.
Note that every time you use an exfoliating product, your skin should be wet so that the product penetrate better.