I've recently started using a price-tracking service called The Traktor. A simple browser extension allows you to track the price changes of products on Amazon, and optionally be alerted when a product drops below a target price.
The tool also embeds price history graphs on the Amazon pages of each product. It's a little bit crazy when you start noticing just how often Amazon changes price. It's not just by the day, it can vary by the hour. For example, consider price changes over the last six months for the Fitbit One activity tracker where the price is almost halved, albeit for a few hours (click image to enlarge).
I guess it can pay to watch those prices closely (as long as you are prepared to wait).