At our house, we love the Pampers Kandoo Foaming products!
To keep from having to buy new bottles when we run out, I simply refill them with our own soap and water. It works great! Once the soap or shampoo is gone, refill with your own clear (not conditioning) liquid hand soap or baby shampoo/wash. Squeeze about 1/2 inch of soap in bottle, then fill the rest of the way with water. Attach lid and give it a shake. The kids will never know!
For more tips, check out Shannon's blog!