I don't know about writing the accent, but I personly don't see 
anything to write about a German accent.  Some of the characters are 
pronounced diferently, but only on German words, so it doesn't apply. 
 I know that there is a little bit of an accent, but being from the 
US, I hear so many accents that it doesn't realy matter. :-)

My suggestion would be to simply listen to yourself talk, and write 
that way.  It can be a bit awkard at first, but it should work.  Also 
you may try using some method to mark the text to tell the readers 
that it is in German, such as <>   _ _ **, or some other symbol.