How can I make sure that my blackberry bushes produce good amounts of fruit?

Do not prune away the floricanes - canes from the previous year's growth - as these are the canes on which fruits will be produced this year. A good wintertime dressing of rich compost or bonemeal provides enough nutrients to promote fruiting in the coming season. Be sure to prune back the floricanes - this year's fruiting canes - after they have born fruit so that the plant can concentrate it's energy on the primacanes - the canes that will produce next year's harvest - for a bigger harvest next season.