It seems this topic comes up fairly regularly. There are several threads on it already, so search for negative intensification, increasing contrast, etc. for a lot more info.
Selenium toning of the negative can get you up to one Zone contrast increase. It is not reversible and won't allow further intensification of the negative, so if you are sure you need more contrast increase than this, don't use it first. Gold toning will likely do the same; I've never tried it on a negative.
Making copy negatives does the job without changing the original negative (as would scanning and printing a digital neg with more contrast... yes, I know: Blasphemy!!).
I tend to use bleach/redevelopment these days for the occasional negative that needs intensifying. Use a potassium ferricyanide and potassium bromide rehalogenating bleach to convert the silver in the negative image into silver bromide. Then redevelop the negative in a staining developer like PMK or Pyrocat. A staining developer is necessary to get a contrast increase. The image silver will return, this time with the addition of the stain, which adds the extra density and contrast. This method works gratifyingly on negatives that have been slightly underexposed, since the shadow detail gets a boost too.
I'll let you search for the details; both here and on the Large Format Forum.
Best,
Doremus