Adding validation to contact.php

Hi
I have created a very simple "Contact Us" php form on my website which is working fine.
However I am now getting quite a lot of empty/spam forms coming back, and would like to add simple validation to ensure that the data fields contain data!
Here is my contact.php form at the moment:

Code:

<?php
$field_name = $_POST['name'];
$field_subject = $_POST['subject'];
$field_email = $_POST['email'];
$field_message = $_POST['message'];

$mail_to = 'richard@richardhorsfield.com';
$subject = 'Website Enquiry from '.$field_name;

$body_message = 'Subject: '.$field_subject."\n";
$body_message = 'Name: '.$field_name."\n";
$body_message .= 'E-mail: '.$field_email."\n";
$body_message .= 'Message: '.$field_message;

$headers = 'From: '.$field_email."\r\n";
$headers .= 'Reply-To: '.$field_email."\r\n";

$mail_status = mail($mail_to, $subject, $body_message, $headers);

if ($mail_status) { ?>
	<script language="javascript" type="text/javascript">
		alert('Thank you for the message. We will contact you shortly.');
		window.location = 'index.html';
	</script>
<?php
}
else { ?>
	<script language="javascript" type="text/javascript">
		alert('Message failed. Please, send an email to richard@richardhorsfield.com');
		window.location = 'contact.html';
	</script>
<?php
}
?>

All I want to do is make all the fields mandatory, so that if they are not completed a message comes up to say "Please enter your email" or something.