I've read somewhere, that if you have more required fields than optional, best practice is to mark the optional form fields.
However I think that asterisk notation for required fields is a widely known pattern. The form will be used by novice/beginner web users, is the above statement (marking the optionals) still better?
Update: @Janel: thanks for clarifying my question, I have to practice this language a little bit more, if I'd like to work abroad later :). It's important however that there are much more required fields than optional (if there would be eg. 3 optional and 4 required field, than I'll definitely mark the required ones - but there is 12 required and 5 optional fields on one page of the form)