1.Read the PDF that guides You through the steps.Its actually quite good and covers most of the important steps.But remember it is not completely up-to-date, as it sometimes tells You to do things that are not on screen, like "Add Profile " when onscreen it says "New Profile", or "Choose File" instead of "Upload File" or stuff like that.
2.Don't follow the video thats on the Development Portal site, as its outdated and will just irritate You.
3.Make sure all Your provisioning profiles are in place AND still valid.Even if Your remote working buddy uses an expired profile, RENEW it, just in case.Make sure Your are using the correct DISTRIBUTION Certificate, and have a DISTRIBUTION PROVISIONING PROFILE.
4.Use Codesign to verify Your myapp.app.zip BEFORE You try to upload it, thats faster and gives You less headache.Open the console, enter codesign -dvvvv or just codesign -dv and drag Your myapp.app (not the *.zip) into the console window.This spits out either a good or bad result.No need to get that message in red from the Application Uploader :)
5.Make sure You build for DISTRIBUTION, not for SIMULATOR.
Hope that helps.Keep watching this site for more info in the future.