Generator brushes wear down. They rub against contacts on the main generator in order to make electricity and this gradually wears them away. They're designed to dot his and are made to be replaced, which is pretty easy, but a bit of a fiddly job. I replaced mine as I thought they had worn down so much the battery couldn't charge any more, but it turned out that the voltage regulator needed replacing. Still, my brushes were quite worn and it did no harm to fit some new ones.
You will need:
There are two generator brushes, one at the top of the generator and the other underneath. If you look / feel down in the gaps in the generator housing, you should see the top one, quite close to the rear of the car. Each brush is held in by a clip which presses it down against the copper contacts on the generator. The wire tail on the brush is screwed in to a contact which transfers the generated electricity to the voltage regulator and then to the battery. It's easiest to tackle the top brush first as you can see what you're doing without getting too twisted up.
First, undo the screw holding the wire of the brush to the contact. Try not to drop the screw, if you do hopefully it will drop through the generator and bounce out of the engine rather than disappear somewhere awkward. Now use the pliers to pull back the clip holding the brush down. Try not to be too harsh with the clip as you don't want to break it - a firm pressure will be enough to pull it up. Use your other hand to tug the old brush free, you can probably pull it up by the attached cable if the brush is loose. Let the clip back down gently in to the gap left by the brush. For the bottom brush it's the same procedure, just a lot more awkward as it's difficult to get your hands in the right place, but at least the screw falls down in to your hand rather than in to the generator. Once the two brushes are out, wipe away any dust and grime they have left behind and have a look at the copper contacts on the generator the brushes rub against. The contacts should be clean and shiny. If they're not you need to clean them. The John Muir book suggests cleaning the contacts by pressing some sandpaper against the contacts with a piece of wood and turning the engine over - causing the generator to spin. My battery was so flat due to the charging problem that I couldn't do this. In the end I rubbed them as best I could and turned the generator by moving the pulley by hand so I could gradually clean the whole way around. I finished the cleaning by giving it a good spray with some car electrical contact cleaner my dad had spare, then leaving it a few minutes to dry.
Fitting the new brush goes like this: make sure the part where the wire contact comes out is pointing towards where it needs to screw in. Lever back the clip using the pliers, you might find the screwdriver helpful for holding the clip levered back. Slot the brush down in to it's holder and ease the clip down on to it. Screw the loose end of the wire down on to the contact using the screw you took out earlier. This is straightforward for the top brush because you can see what you're doing. The bottom brush is much harder as you can't see what you need to most of the time. You might want to make sure anyone who is easily offended is out of earshot while you do the bottom one so you can swear as much as you like. Once the brushes are in you can start the car, knowing you won't need to do this job again for another several thousand miles.