ช่วยดูให้หน่อยคับ Warning: Cannot modify header information - headers already sent by

<?
	
	session_start();
	include '../connect/connect.php';
	$strSQL = "SELECT * FROM account WHERE Username = '".trim($_POST['txtUsername'])."' 
	and Password = '".trim($_POST['txtPassword'])."'";
	$objQuery = mysql_query($strSQL);
	$objResult = mysql_fetch_array($objQuery);
	if(!$objResult)
	{
			echo "Username and Password Incorrect!";
	}
	else
	{
			$_SESSION["UserID"] = $objResult["UserID"];

			session_write_close();
			
			{
			header("Location:index.php") ;

			}
	}
?>