The submit function should reset the form after every user as part of the process. But if that doesnt work you have another option the easiest is to just create a reset button so you or the audience can reset the form at any time after the submission. This would be coded like this.
<form action="url-to-formmail-provided-by-your-ISP" method="post">
First name: <input type="text" name="firstname" /> <br />
Surname: <input type="text" name="surname" /><br />
<input type="reset" value="Clear form" />
<input type="submit" value="Submit now" />
This would give you a submit and reset buttons.