I have a form with a standard reset button coded thusly:
Trouble i
A method I used on a fairly large form (50+ fields) was to just reload the form with AJAX, basically making a call back to the server and just returning the fields with their default values. This made is much easier than trying to grab each field with JS and then setting it to it's default value. It also allowed to me to keep the default values in one place--the server's code. On this site, there were also some different defaults depending on the settings for the account and therefore I didn't have to worry about sending these to JS. The only small issue I had to deal with were some suggest fields that required initialization after the AJAX call, but not a big deal.