I have to chime in here as well:
First, whether you can increase contrast by "tweaking" the development depends on a couple of things. 1) The paper: some papers just do not react well to developer manipulations. 2) The developer you use as "standard." Dektol 1:2 is fairly contrasty already. You can try full strength, adding carbonate/Kbr, increasing dev time etc. and see if it works, but unless you are developing for less than 2 1/2 minutes in the Dektol, you may not see much change. (Getting less contrast would be a lot easier...)
Second, much of what is perceived as increased contrast when "tweaking" developer is really a change in characteristic curve shape that gives more contrast to a local area, but not a real change in overall contrast. This, I believe, is the advantage to playing around with your developers. However, it is possible to come up with a steeper overall curve, which is increased contrast as far as papers are concerned. (As opposed to films, which are rarely developed to d-max, the d-max and highlight density of papers stay relatively constant. It is the curve shape that determines the range of negative densities that can be printed on a given paper and thus the "contrast" of the paper.)
Three, selenium toning the negative will give the negative a real contrast increase. However, if you developed the negative in a staining developer (e.g. PMK), the toning solution will remove the stain, effectively negating the contrast increase. You may be able to recover the stain by bathing the neg in spent developer or an alkaline solution after the toning, but I am not sure, as I haven't tried it myself.
Selenium toning your print will increase d-max and contrast overall. However, the highlights are least affected. Sometimes, printing a bit lighter and selenium toning is just the ticket, however, bringing more life to the highlights than one might expect.
My recommendations: First, old paper loses contrast. Try a fresh batch of paper if yours has been around for a while. I have some old Seagull grade 3 that is down to about grade 2 now.... Try toning if you aren't already.
If you really need an overall contrast increase, try selenium intensification of the neg (as long as it wasn't developed in a staining developer). It really works and can be done locally as well. Selenium toner 1:2 for 5 minutes is toning to completion and approx. a one-grade increase (maybe a bit less). Less time will give you less contrast change. This is a proportional increase and usually very gratifying when it works. These are the "easy fixes" and will give your prints more snap.
If you want to play with the developer, try:
1) Full strength Dektol with an extended development time (5 minutes is a good starting point) and reduced exposure to compensate. This can give a darker, heavier print. If you want more highlight contrast, but the blacks are fine, use the standard dilution and...
2) add carbonate and/or KBr (or benzatriazole) to your developer. Adding carbonate alone increases developer activity and is somewhat like inreasing developer strength. It affects the blacks mostly. KBr restrains development in the the highlights and can, with increased development/exposure time, give a print with more highlight separation. You can use both together to balance each other somewhat and get some darker blacks and more highlight separation at the same time. I keep pre-mixed solutions of carbonate and KBr on a shelf above the developer for easy "tweaking." I usually just pour some in, but you can measure if you like. There is a good discussion of using carbonate and KBr in the Darkroom Cookbook. You will have to adjust exposure and/or development time when "tweaking." Again, some papers react better to these techniques than others.
You can bleach up the highlights by immersing the entire print in a potassium ferricyanide solution. You can also selectively bleach certain areas of the print. Bleaching techniques are a bit complicated to go into here, but there is lots of info on the web. If you haven't bleached before, take time to master the techniques as they can be tricky.
Bleaching affects the way the print will tone later, so if you tone, especially with selenium, be aware that split toning can result. When I bleach locally, I use a very dilute toning solution and watch very carefully so I can pull the print before split toning occurs. I usually end up wasting the first one...
If none of the above options works, you need to find a contrastier paper. Maybe you will have to resort to VC or a different brand of graded that still comes in grade 4 (Seagull, Kentmere, ???). As a last resort, there are intensifiers for the neg. So far, I have managed to get by without having to resort to more intensification than selenium offers, so I can't offer first-hand assistance with that.
Hope this helps,
Doremus Scudder
www.DoremusScudder.com