I must say intensification is always to be used when there isn't anything that can be done with a negative.
It works better on underdeveloped negatives (if I remember).
It won't add image where there is none. It has a tendency to make the image somewhat grainy as the grain tends to clump together.
Printing on grade 5 is to be the first step, like Leon said. Intensification will make the negative more dense by adding metal to the weak image.
You do have to read carefully before choosing which intensifier to use as some work proportionally while others don't.
The cool thing about the process is that it can be done under normal light so you see exactly what you're doing. When it's dense enough for your taste, you just pull it out and follow the remainder of the process (fix and rinse if I remember). There is always a limit to how much more density you will get!
As for Negative Selenium intensification... this is a weird trick I heard works.
You know, then you tone a print in Selenium, it gets a bit darker... Well, it does the same thing to a negative (it is silver after all, just on transparent base instead of paper).
It makes the image more dense and stable.
But this is a trick... you could do a web search for it. But this is roughly what you'll find:
To intensify negatives that are lacking in contrast, first soak them in a wetting agent such as Kodak Photo-Flo or Fuji Driwel and then immerse them in Kodak rapid selenium toner diluted 1:4. The maximum toning effect is acquired after about six minutes. Then run the negatives through a clearing bath and wash them thoroughly.
If you're curious, you could always try it on a scrap negative. If it gives you the effect you're looking for, then you will know what to do if the same situation arises.
Good luck.
