Lots of useful info here http://www.rosemagazine.com/articles02/pages/blackspot.asp and buying resistent varieties certainly makes a difference.
Sadly one of my favourites, Mme Alfred Carriere, has proved to be very susceptible.
As has been said, garden hygiene is a good preventative - this means picking up and burning all affected leaves when the fall off the bush.
Mulching over winter and spring helps prevent the spores from reinfecting the plant, and don't forget that a strong and healthy plant will have much better resistance to the effects of black spot, so when you plant your roses prepare the soil well and remember to feed them.
And on the bright side, the reason black spot is more prevalent than it was in our parents' youth is because of the improved quality of the air we breathe - when it was full of sulphur and smoke from coal fires the black spot fungus couldn't cope