Good place to start is http://www.blu-ray.com
At the top of the menu screen is a list of movie titles (scroll all the way to the top). It's the "Latest blu ray price drops", or you can switch to "Top Deals".
Also there's a red menu button at the very top "Best Blu-Ray Deals, See All the Deals"
and it shows prices of blu rays on Amazon, listed in several categories (lowest to highest, 3d br, 2d br, latest price drops, etc...).
Also, if you click on any BR title you can see a little graph above the pricetag listing. Move your cursor over it, and a window pops up showing the price history of that title.
Wait around for good deals on titles that interest you.
Also, the reviews of the BRs are reliable. Included are grades for video/audio quality. I agree most of the time with their assessments.
If you wait 3-6 months for new release titles, they can drop MASSIVELY. For example last month MISSION: IMPOSSIBLE GHOST PROTOCOL was $9.99.
Hope that helps, and happy searching!