Here is a quick, easy way to add a centered guide to any Photoshop document:
Choose View > New Guide, and enter a position of 50%.
That’s it. Want to mark the center of your document? Repeat the process twice with both horizontal and vertical guides. This technique works for all percentages, so if you want a guide at 1/3 the width of your image, just enter a percentage of 33%.
Source: This tip surfaced during the audience Q&A after the first class in our 10-week Photoshop Course. I always learn something new in even the most basic class that Jason Hoppe teaches.