The best approach for me has been to use a function to log the values to wp-content/debug.log, lifted from http://fuelyourcoding.com/simple-debugging-with-wordpress:

if(!function_exists('log_it')){ function log_it( $message ) {   if( WP_DEBUG === true ){     if( is_array( $message ) || is_object( $message ) ){       error_log( print_r( $message, true ) );     } else {       error_log( $message );     }   } }}

Then use the function like this in your save_post hook:

log_it($_POST);log_it('The value for ' . $custom_field . ' is ' . $_POST[$custom_field]);

Make sure that wp-content/debug.log is writable, and that you have debugging enabled in wp-config.php:

@ini_set('display_errors',0);define( 'WP_DEBUG',         true );  // Turn debugging ONdefine( 'WP_DEBUG_DISPLAY', false ); // Turn forced display OFFdefine( 'WP_DEBUG_LOG',     true );  // Turn logging to wp-content/debug.log ONdefine( 'WP_POST_REVISIONS', false); // Disables revision functionality

Method 1:

if (isset($_POST)) die(print_r($_POST)); //answer by Tumas

Method 2:

create log file (my_logs.txt) in a folder, where you use this code:

add_action('save_post', 'something_process',11,11);function something_process() {    print_r($_POST);    $tmp = fopen(dirname(__file__).'/my_logs.txt', "a+"); fwrite($tmp,"\r\n\r\n".ob_get_contents());fclose($tmp);}

The best solution I've found so far is storing the $_POST in a session var for access later.