Another way to intensify a negative is to bleach and then redevelop in a staining developer. This process can be repeated, with the stain building up with each repetition (though there comes a point where grain growth from the repetition can overcome any benefit of the intensification).

Any other method than selenium toner should be tried before that step; once you've selenium toned the image, it's impervious to most other intensification processes (because the silver selenide layer on the image silver particles doesn't react with common image bleaches).