Two weeks ago, I pinched off the tops of the black raspberry canes.  Left to their own devices, black raspberry primocanes will grow so long they bend down and root a new plant at their tip.  This trait is useful if you're looking to expand your berry patch, but is less useful if you actually want to be able to get into the patch to pick berries.

If you choose to keep your plants contained, you can pinch off the tips when the brambles reach waist high.  In most plants, apical dominance tells the plant to put all of its energy into growing the main shoot, but if that main shoot is gone, the side buds are allowed to grow.  The bottom photo shows what one plant looks like two weeks after pinching.  Notice how the side shoots have grown out --- these will all be coated with fruits next year when the primocane becomes a floricane.

I've similarly pruned our cultivated blackberries.  Blackberries usually aren't tip-rooters, but mine are, and I've also found that they will grow primocanes a dozen feet long if left to their own devices.  I prefer the plants to stay contained in my obsessive rows, so I take a few minutes to summer prune.