Add these lines:

ini_set( 'display_errors', 0 );
ini_set( 'log_errors', 1 );
ini_set( 'error_log’, WP_CONTENT_DIR . '/debug.log' );

Although the use of WP_CONTENT_DIR doesn't work for me, I did this instead.

ini_set( 'error_log', dirname(__FILE__) . '/wp-content/debug.log' );

Also, since I don't want all those silly notices and warning clogging up my debug.log file, I just want errors only, I do this:

ini_set( 'error_reporting', E_ALL ^ E_NOTICE );

This line basically tells the php parser to report all errors, except notices. If more plugin programmers would test their plugins with the notices on then I wouldnt have 3 pages of notices come up inside the debug.log file.

So in summary:

ini_set( 'display_errors', 0 );
ini_set( 'log_errors', 1 );
ini_set( 'error_log', dirname(__FILE__) . '/wp-content/debug.log' );
ini_set( 'error_reporting', E_ALL ^ E_NOTICE );

Based on the comments section, this is a more proper route to go then actually using the defines that were written into the code, dunno why but I take people who know the wordpress core code better then me at their word. Thanks guys for great comments.

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Then if you navigate to your server's wp-content directory there will be a debug.log waiting for you with information in it about what is wrong with your wordpress if an error ever happens.

/wp-content/debug.log

PHP.Net Manual on ini_set: http://www.php.net/ini_set

- Phil (Frumph)