Disconnect everything you *can* (looking at you, Logitech speakers) and take in the slack. Make tiny bundles and wrap them with velcro cable ties. Zip ties are nice, but they're a bitch if you need to move something or screw up.

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If you want to get crazy, grab some 3M command strips or dual lock and mount the power strip to the under side of the desk.

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I also second the SIGNUM. At the VERY least, it keeps the bundle underneath the desk rather than behind.