Compile, Build or Archive problems with Xcode 4 (and dependencies)

NB: The steps below will solve 90% of your Xcode archive issues however, from the comments it is suggested you try quitting Xcode first. This may save you hours of setting tweaking.

1. Check the "user header paths" are correct (Add "" to paths for spaces, both in your project and dependencies)
2. Set "Always search user paths" to YES
3. Create a group call "Indexing headers" in your project and drag the headers to this group, DO NOT add to any targets when prompted. This includes any headers inside your .xcdatamodeld, you'll need to right-click and view package contents to find them.
4. For all dependencies set "Skip Install" build setting to "Yes"
5. Moving any "Public" headers in Build Phases to "Project"
6. Set the Build Setting "Installation Directory" on your Target to $(LOCAL_APPS_DIR)
7. Change the target build setting "scan all source files for includes" to YES. (link)
8. With newer versions of Xcode (> 4.2) you might want to read this question related to workspaces.
9. Manually delete the project.xcworkspace files form all referenced projects

I had the same problem in XCode 4: "Lexical or preprocessor issue MyFile.h not found". However, MyFile.m was not a static library, just a standard class. And MyFile.m and MyFile.h were included properly and indexed in the project.

So ... I quit XCode and the Simulator, then restarted them and the problem disappeared.

I found that the problem went away when I changed the target build setting "scan all source files for includes" from no to yes.