I had the 'header already sent' error message several times on my pages. I found that if I wrote my includes/requires after the "header('Location....') then it worked.

For example, I had:
<?php
require("config.php");
require("userheader.php");
requrie("usernav.php");

*some more code*

header('Location: home1.php');
?>

If I put the 2nd and 3rd requires at the end of the php code (after the whole script) then I didn't receive any errors. That's what I found anyway :)