To quickly set guides in the center of any size and/or resolution Photoshop document, set the Preferences for Units & Rulers to Percentage, show your Rulers (Command (Mac) / Control (Win) + R), and record an action dragging guides to the 50% marks. (You can also select View > New Guide to place the guide at the 50% mark instead of dragging it out from the rulers.) Next time you need guides, simply play the action. For more information about creating actions, please see:
Basic Actions and Batch Processing
In this episode of The Complete Picture, learn how you can accomplish more in less time using actions and batch processing in PSCS3’s to streamline your workflow and minimize repetitive tasks.
Advanced Actions, Droplets and Scripts
In this episode of The Complete Picture, take automation to a new level in PSCS3 by mastering complex actions in order to turn several steps into a single click. Discover droplets, integration with LR and scripts.

Oh so simple yet SO useful! Thanks a bunch!!