When aligning layers, it’s best to turn on Smart Guides. Use the Move tool to reposition the layers - Smart Guides automatically appear when they are aligned at top, center, bottom, left or right.
To change the image size of a document and have the guides in the image resize proportionally, be sure to unlock them by unchecking the View > Lock Guides menu item. If you need them to stay in place, then lock the guides before resizing.
Place a guide numerically using View / New Guide and Lock using View / Lock Guides or Command + Option (Mac) / Control + Alt (Win) + “;” . And finally, when dragging out a guide from the rulers, Option (Mac) / Alt (Win) toggles the orientation of the guide (vertical to horizontal).
To change the image size of a document and have the guides in the image resize proportionally, be sure to unlock them by unchecking the View > Lock Guides menu item. If you need them to stay in place, then lock the guides before resizing.
To reposition a guide using the Move tool, position the Move tool directly on top of the guide. When the icon changes to a double headed arrow, click and drag to reposition the guide. Drag a guide outside of the image area to quickly delete it. Shift-drag a guide to snap it to the ruler tic marks even is the “snap to” option is turned off (View / Snap To...).
Use Preferences / Guides, Grid & Slices to control the Style, Color, and Grid line spacing and subdivisions. Yes, this means that guides can be a soothing neutral gray, not Cyan or Magenta!
To quickly display rulers at the top and left of an image, click the View Extras icon in the new Application Bar in Photoshop CS4 and select Show Rulers from the drop down menu, or use the keyboard shortcut, Command (Mac) / Control (Win) + R to toggle the rulers visibility. Command (Mac) / Control (Win) + “;” toggles the visibility of guides and Command (Mac) / Control (Win) + ‘ toggles visibility of grids.