The answer depends on whether you are distributing a Web form or a PDF form.
In a Web form there is no way to insert a button, but there is a feature that will redirect the form back to a clean version of itself after a user clicks the Submit button. This feature is enabled in the Options tab. Go to Options-> Redirect URL and select the Reload the form option.
For a PDF form you can add a button if you have access to Adobe Acrobat Pro. What you will be doing is saving your form as PDF from FormsCentral without adding a submit button then editing the form in Acrobat Pro. You will then import the PDF with the clear button back into FormsCentral and re-save the file WITH the submit button.
Steps for how to save the PDF file from FormsCentral are at: http://forums.adobe.com/docs/DOC-3661
A video on how to add a reset button to a PDF is at: http://tv.adobe.com/watch/acrobat-x-tips-tricks/quick-tip-how-to-add-a -reset-button-to-a-form/
I hope this helps.
Software Quality Engineer
Adobe Systems, Inc.