We start outside the Photoshop Document, with some good ol' external file organization. I mean, if you can't get your files organized in a reasonable way, what would anything else that you do inside the PSD matter?
WHY? Keeping everything in one PSD reduces the confusion of which file to use.
WHY? Ironically, absolutes like “Newest” and “Latest” are bound to have edits later. It's best to just keep a general file name.
WHY? PSD inheritors shouldn't have to go hunting to find original assets. Proximity is key to avoiding wasted time.
WHY? It's better to have control over how the UI elements look than using browser defaults. You're most likely going to be asked to design them at some point anyways.
WHY? Having to be asked where you put a file is common, but it shouldn't be necessary if you learn to put it on a file share before even being asked.
Always Organize Your Files
Arguably the biggest source of angst amongst PSD inheritors, messy and unnamed layers suck the fun out of any situation. If you do nothing else, name your layers!
WHY? The quintessential piece of etiquette: naming your layers instantly decreases confusion and improves acclimation to an otherwise unfamiliar document.
WHY? Happy layers have happy homes. Folders group like-items for easier navigation and editing.
WHY? Having too many layers hanging around usually results in an unnecessary bloat of file size and can affect performance.
WHY? If you use an element (logo, photo, etc.) in multiple layouts within the PSD, only having to edit it in one place makes a lot of sense.
WHY? Layer Comps & Smart Objects offer a way of showing multiple states or layouts without needing to build (and maintain) another PSD.
Always Name Your Layers
Working with images is all about being non-destructive. You certainly don’t want to limit the shelf life of a photo or button by recklessly adding irreparable filters or effects.
WHY? It's always better to only have to edit one mask than multiple masks.
WHY? When you’re consistently off by a pixel, somebody has to go in and align things correctly. Shouldn’t it be you?
WHY? It’s difficult to figure out what the hex value of your mashed-up color is. Make it easy.
WHY? If a silhouette needs retouching, or a logo needs resizing, or you choose to go back to full color photos instead of black and white, it’s much easier when it’s not erased, rasterized or flattened.
Always Stay Flexible
There’s no getting around using type in Photoshop, no matter how dissimilar the text rendering is to what happens in browser. What you can control, however, are the values and areas you use text.
WHY? Consistency in your typesetting is key. It rarely makes sense to use fractions and decimals in Photoshop.
WHY? There's nothing worse than needing to edit type and not being able to because you don’t have the fonts.
WHY? Beyond not being able to do it programmatically, I doubt the typographer intended for their type to be squished or stretched.
WHY? Longer-than-necessary text boxes can prohibit selecting text that’s behind it and can be frustrating to work around.
WHY? It takes more clicks to find the font family and size of multiple types of text within one text box than it does to do so on an individualized, singular text box.
Always Be in Control
A plea: please use effects and filters in moderation. PSD inheritors will have less to sort through, and in most cases, more filters ≠ better design.
WHY? It’s much easier to navigate the layer panel with color thumbnails that represent the color of the elements on the canvas.
WHY? Without knowing how an image translates to CSS, you could be greatly limiting its use by loading it up with effects.
WHY? Gradients are meant to be smooth, and like everything else, it’s better to get it right the first time.
WHY? Using outside strokes when you really meant to use inside strokes could affect the overall width of a shape.
Always Think Before Using Effects
Not that any of the following guidelines will automagically make you a better designer, but they'll help your approach to working in Photoshop for certain. Just five of the many considerations you can be making.
WHY? Grids help guide (pun intended). Establishing a system of heights, widths and alignment for your design is essential.
WHY? Drop shadows, like other effects, need to be altered from the Photoshop default settings in order to look realistic or sophisticated.
WHY? Besides the amazing selection to choose from, webfonts are the norm. Not the norm? Text as images.
WHY? It’s best to design with the mindset of how things will adapt to different widths, as opposed to hoping everything plays nice after the fact.
WHY? Don’t risk getting caught using unauthorized photos or icons. People worked hard to produce nice things for us. We simply need to fork over a few bucks.
Always Keep Learning
Ever hear of the term “being too close to a design”? While knee-deep in pixels, it’s easy to miss some glaring mistakes. Employ some quality assurance.
WHY? It’s easy for a designer to overlook things like misspelled headlines when you're focused on individual pixels.
WHY? It’s easy to go in to auto-pilot when you’re in Photoshop. Just make sure you’re not forgetting anything that was originally agreed upon in the wireframes.
WHY? The embarrassment. Oh, the embarrassment (plus it's illegal).
WHY? Browsers each render content differently, but more importantly, there are some that don’t take advantage of CSS3, or basic PNG transparency (ahem, IE6).
WHY? Unless intentional, it’s painstaking to have to eyedrop 10 different blues all trying to be the same value, but aren’t.
Always Be Consistently Thorough
One thing’s for certain: if your job is to export images from your PSD, it’s something you only want to have to do once. Don’t overlook this step in the process.
WHY? Save for Web (& Devices) is geared towards properly compressing your image to be used on the web. “Save As” is not.
WHY? With your images being delivered over all kinds of networks to all kinds of devices, proper compression and small file sizes are incredibly important.
WHY? This goes along with “Name Files Appropriately”, since the goal is to be as clear as possible with what the file is, as simply as possible.
WHY? CSS excels at being easy to adjust positioning. Photoshop's workflow (Open --> Nudge --> Save for Web)? Not so much.
WHY? iOS devices are almost exclusively “Retina”. More devices are sure to follow.
Always Take Your Time