Note to self: Vote for BenH’s answer. It’s perfect.

First of all, you shouldn’t be developing anything important unless you have your procedure down perfectly. I’d suggest, first of all, before developing these important rolls, you expose several more rolls with unimportant stuff. Film is cheap, but experience is invaluable.

The “lightness” of the negative is commonly called the density. There are four factors which control density.

Probably the most important of these factors is exposure. Basically, for a given lighting situation, the more exposure you give the negative, the more dense it will be.

The other three factors controlling density, in no particular order, are time, temperature, and agitation. Increasing any of these will increase density to a point. Keep in mind however, that they really do nothing for the maximum density, dMax, however they do help bump some grains out of the lowest part of the curve, the dMin. Thus, increasing time/temperature/agitation also increases contrast, which often is undesirable.

In any case, I’d suggest shooting a test roll, as I mentioned. Then, when you go to develop it, take the time and temperature you’ve been using for the developer as a starting point. Keep the temperature the same(and make sure the developer is fully and uniformly that temperature), but increase your developing time by 10%. For agitation, I suggest continuous inversion for the first thirty seconds, and then one single inversion every 30 seconds thereafter. Also, remember that development time starts when you start pouring the developer in, and stops when you begin pouring the stop bath in.

If a 10% increase in time doesn’t work, shoot another test roll and increase the developer time by another 10%. Keep doing this until you get negatives you’re happy with, and then you should be ready for your important rolls.

Three other things:
If your development time is under 5 minutes, decrease your temperature to such a point as you’re able to extend the time past 5 minutes. Times under 5 minutes tend to give uneven development.

You will hear varying opinions on the necessity of prewetting the film, however I feel that it’s greatly beneficial and suggest always doing it. Prewetting the film allows the emulsion to absorb developer more quickly and promotes more even development.

Remember that the developer is the only temperature critical step of the process. Don’t be afraid of spending too much time in the fixer. Fixing is a process to completion step, and it’s extremely important that all of the undeveloped silver halides are removed to prevent problems down the road. Don’t be afraid to leave the film in the fixer for 15 or 20 minutes.

As for printing your thin negatives, there are ways. As a starting point, I’d suggest using a nice long exposure and using a higher contrast filter than you would normally use for printing. This will definitely take some experimenting, although it should certainly be possible to get a decent print. It’s always best to start with a good negative, though, which is why I suggest getting your process down before doing any important work.