Just like most page-layout applications, Photoshop has non-printing guides you can pull out anytime you need to align objects or type, but there’s also a trick for flipping the guides. To access the guides, make your rulers visible by pressing Command-R (PC: Control-R), then click-and-hold within one of the rulers and drag out a guide. If you pull out a horizontal guide from the top ruler, but really wanted a vertical guide, just press the Option key (PC: Alt key) as you drag and your guide will flip from horizontal to vertical (pretty slick). You can pull out as many guides as you need (there’s probably a limit to how many you can use, but we’ve never reached it). When you’re done using a guide, just use the Move tool (V) to drag it back to the ruler where it came from. To remove all of your guides at once, choose Clear Guides from the View menu.