This arstechnica article describes the basic steps:
Start by visiting the program portal and make sure that your developer certificate is up to date. It expires every six months and, if you haven't requested that a new one be issued, you cannot submit software to App Store. For most people experiencing the "pink upload of doom," though, their certificates are already valid. What next?
Open your Xcode project and check that you've set the active SDK to one of the device choices, like Device - 2.2. Accidentally leaving the build settings to Simulator can be a big reason for the pink rejection. And that happens more often than many developers would care to admit.
Next, make sure that you've chosen a build configuration that uses your distribution (not your developer) certificate. Check this by double-clicking on your target in the Groups & Files column on the left of the project window. The Target Info window will open. Click the Build tab and review your Code Signing Identity. It should be iPhone Distribution: followed by your name or company name.
You may also want to confirm your application identifier in the Properties tab. Most likely, you'll have set the identifier properly when debugging with your developer certificate, but it never hurts to check.
The top-left of your project window also confirms your settings and configuration. It should read something like "Device - 2.2 | Distribution". This shows you the active SDK and configuration.
If your settings are correct but you still aren't getting that upload finished properly, clean your builds. Choose Build > Clean (Command-Shift-K) and click Clean. Alternatively, you can manually trash the build folder in your Project from Finder. Once you've cleaned, build again fresh.
If this does not produce an app that when zipped properly loads to iTunes Connect, quit and relaunch Xcode. I'm not kidding. This one simple trick solves more signing problems and "pink rejections of doom" than any other solution already mentioned.