First, it depends on the shape of the hull.
Sometimes you need to start by wet sanding, and then progressivly work up to buffing. If it's in bad shape, you may need to do a few different grades of wet sanding, starting low, perhaps 400 or 600 grit, then progressing up to 2,000 grit before you start buffing.
You can also then move to a medium grade of buffing compound such as 3M Perfect it or 3M Super Duty rubbing compond, depending on how things look.
But if it's a medium to light haze on the fiberglass, or as a final step, I have had great results on a black hull (that willl easily show any defects) using 3M's 'Finesse-it II', the results end up looking like a mirror finish:
http://www.westmarine.com/buy/3m--fi...04_130_001_511
I've used both the 'blue' foam pads, or the recommended wool pad with a rotary buffer at 1500 rpm. Just read the directions.
After this I follow up with a good marine wax, last year I used Mequires flagship wax:
http://www.westmarine.com/buy/meguia...e-wax--9309501
I've put this on by hand, or using an automotive type orbital buffer, then buff off with terry cloth pads with the same orbital buffer.
Note you can find these a lot cheaper on Amazon than West Marine, but the West Marine links are easy to get to. As an example, I got a gallon of the finesse-it-ii last year via an Amazon sale, for not much more than what West Marine charges for a quart.
I'm sure others will have their favorite products. I hear people talk of 'buf-magic' and other things. I believe most of these products are similar and you can use whatever brand(s) are your favorite.
Just be careful and don't stop moving with a buffer, on some boats the gell coat may not be very thick, and you don't want to buff through it!