Pursuant to article 15 of Law 3068/2002 you have a right to appeal your visa denial before a Greek administrative Court (the appeal may be submitted through a Greek Authority - embassy) within ninety days of the date on which you have been notified of your visa denial. If you appeal through the court, then the appeal must be written in Greek. Here is a sample Schengen visa appeal (template):
Schengen visa appeal (scroll down for a PDF)
http://www.euro-dollar-currency.com/schengen_visa_appeal.htmThis post might be handy to you too as it provides a template for a visa invitation letter:
Schengen visa invitation letterhttp://www.maxi-pedia.com/forum/index.php?topic=942This list might give you a good idea, perhaps you have forgotten some of this documentation in your initial visa application?
Basic requirements for Schengen visa applicationhttp://www.maxi-pedia.com/forum/index.php?topic=977.msg2646#msg2646You did not mention in your post the reason why your visa was denied, in case it was the commonly issued "The information provided about the purpose and the conditions of intended stay was not reliable", then this post might be helpful for you too:
The information provided about the purpose and the conditions of intended stay was not reliable(visa supporting documents)
http://www.maxi-pedia.com/forum/index.php?topic=632Let us know how your case proceeds, whether you were successful.
Cheers.