I don't know if this might be of any use for you, but I recognise the errors you receive as being very annoying.
Basically what this error says is that there has already been some HTML output, so it's too late for the PHP sessions to be checked (or something like that, correct me if I'm wrong).
What I always have to do when fixing this error is to make sure there is NO HTML output what so ever before the session bit, which means that all blank lines have to be removed. If there is a blank line on top of your file: delete it. I know it sounds silly, but hey. It works for me
Hope that helps!