Custom variables, such as those you add via Manage -> Forms can be accessed using the name that you gave them and the prefix the the built in form that you added them to. So if you added a field called Agency that was added to the built in "Ticket Details" you would access it as ticket.agency, aka:
%{ticket.VARIABLE}
Data associated with "Contact Information" is available via %{ticket.user.VARIABLE}. Where VARIABLE is the variable name that you assigned the field in the form. Data associated with other forms (such as extra forms added to help topics) is currently not available this way.
source: http://tmib.net/osticket-19x-email-template-variables
I do not think that this information has changed since I wrote the article.