Remove enough from the top to let light down into the plant center. Cut out the weakest canes. Sometimes it is better to cut the cane back to a strong new side branch, rather than pruning it back to the ground. If your plants tend to overbear, thin the fruit buds by tipping back some of the small shoots carrying a heavy load of flower buds. Pruning severity may need to be adjusted to balance the production of a good, quality crop with adequate growth for subsequent years' crops.
Experience is the best guide for proper pruning.
Weak bushes require more pruning than vigorous bushes because pruning stimulates vegetative growth. Special consideration must be given to cultivars with spreading habits. Sprawling canes should be removed, but care should be taken to leave sufficient canes for fruiting. When rejuvenating an old planting, remove one of every 5 or 6 canes. In following years, remove up to 20 percent of the wood until new cane growth occurs.