That depends on whether you are distributing a web/HTML form or a PDF.
For the web form there isn't a "Clear form" button you can add - but you can set it up so that after Submit the form "resets" automatically. This is a feature in the paid level subscriptions. There is a "Redirect URL" sub tab in the Options tab of the form, the "Reload the form (redirect to the first page)" option will reload and clear the form.
For the PDF, if you have access to Adobe Acrobat you can edit the PDF after saving from FormsCentral and add a Clear form button.
Here is a tutorial on adding the button: http://tv.adobe.com/watch/acrobat-x-tips-tricks/quick-tip-how-to-add-a-reset-button-to-a-f orm/
You would also want to review this FAQ on editing a FormsCentral PDF in Acrobat: http://forums.adobe.com/docs/DOC-3661