Of course, I decided to take on this chore on the hottest day of the year. I quickly learned that when cutting out the big thorny canes, you should wear the toughest pair of leather gloves you have. I have five blackberry plants, and each already had new primocanes 3-6 feet tall, so I had to be careful not to damage those while removing the old ones. I got all the old canes cleared out, and selected the best looking new primocanes, removing the others. I also cut out a lot of 'volunteers' that were popping up all over the place.
Once I got everything cleared out, I tied the canes I wanted to keep to the posts, and cut the tallest ones to 5 feet. Last year, I tied up the canes with green plastic garden tape. Unfortunately for me, as the canes got larger, their thorns would just puncture and cut the tape, and the cane would fall over. I had to tie and re-tie them several times over the course of the year. This year, I'm using a combination of green tape for initial guidance, and plastic cable ties for reinforcement.
Finally, I used the weed whacker to trim down the grass growing in the old mulch, put down a good bit of cottonseed meal, applied a layer of newspaper and cardboard, and spread 3-4 inches of mulch. I gave them an hour of water from the soaker hoses. I'll have to prune them and train them throughout the summer to encourage lots of side branches, which will mean lots of berries next summer.