As an app developer myself, I've (recently) just began using ImageAlpha + ImageOptim (http://pngmini.com/) to compress all my PNG images. These really do the job better than PNGCrush (Xcode's default) and cut them down to 50% of the original size without losing much quality.

From one app developer to another, if you're adding retina assets for the new iPad like me, remember to use ImageAlpha first!

P.S. If you're unsure of its advantages, check out this case study of ImageAlpha + ImageOptim used on Tweetbot: http://imageoptim.com/tweetbot.html