Mac: We've seen a few different utilities that help you clean up your iOS device's storage and make space for new files, but PhoneExpander is one of the easiest to use. It's also free while it's in beta.
When you connect your device to your Mac, PhoneExpander takes a look at your storage space and helps you clean everything up. You can clear out cache from various apps, backup photos to your Mac, and track down any big apps. Eventually, you'll also be able to clean up your music library too.
It works similarly to previously mentioned PhoneClean, but offers a little more control over what you actually keep around. That said, it is in beta still, so while we didn't have any problems using it, there's always the potential for little hiccups along the way.