All you need to do is use the name of your field in square brackets.
For example, if you've got a form such as:
<p>Your Name (required)<br />
[text* your-name] </p>
<p>Your Email (required)<br />
[email* your-email] </p>
<p>Subject<br />
[text your-subject 80/] </p>
<p>Your Message<br />
[textarea your-message 80x10] </p>
<p>[submit "Send"]</p>
Then you can refer to these fields in the message body as follows. Use the same field names as you used in the form, you can use any field names you like.
From: [your-name] <[your-email]>
Subject: [your-subject]
Message Body: [your-message]