Code completion not working in Xcode 4?

I have been suffering from this bug since I have switched to 4. I found two things that have helped:

1. Make sure your header paths are absolute.You can prefix your relative paths with $(SOURCE_ROOT) to make this work

$(SOURCE_ROOT)/../path/to/some/headers

2. Clear your derived data. In Xcode, Select Window, then Organizer. Select your project and select Delete on Derived Data.

I found it necessary to close and re-open my project in order to see completion working again.