Flushing a stringstream

Basically, when print something to the stringstream and later extract it it still remains in the stringstream. I want to flush the stream so the next insertion I make will only be extracted.

stringstream::flush() isn't working. Please help!

Code:

   std::ifstream keywords("keywords.txt", std::ios::in);
   if (keywords.is_open())
   {
      std::string line, fmtkey;
      std::stringstream format;
      while (std::getline(keywords, line))
      {  
         format << ' ' << line << ' ';
         format >> fmtkey;

         // How do I flush the entire stream? Contents from last insertion/extraction still remain

         format << "<layer class=\"keyword\"> " << line << " </layer>";
         format >> line;

         special.push_back(stringpair(fmtkey, line));
      }
      keywords.close();
   }

Nevermind, I figured it out.
Thanks Eibro.