I am not the world's most attentive rose grower (who can focus on roses when there are tomatoes forming??) but I do try to keep an eye on the few rosebushes I grow for signs of blackspot. Blackspot seems to be a yearly problem for me, though it seems to be less serious as the years go by. This is probably because I've become more vigilant in recent years about removing spotted and fallen foliage right away -- leaving diseased foliage in the garden is practically a guarantee that you'll have the problem again next year.
Besides keeping the garden tidy, it's a good idea to practice proper watering techniques (avoiding overhead watering when possible). Most of the other tips for preventing and controlling blackspot on roses relate to proper placement of your plants in the first place -- such as siting them in full sun and making sure they have ample air circulation.
But the best way to prevent blackspot in the first place is to plant resistant varieties. Some popular blackspot resistant roses can be found in this extensive list from Purdue University.
Do you grow any roses that you've found to be particularly resistant to blackspot? I grow 'Belinda's Dream," and have found it to be very care-free in this regard.