Be pixel perfect
Wherever possible, try to use vector shapes for all shapes in your design. This may seem obvious but some designers still draw boxes using the Marquee tool and fill. A common problem with vector shapes is edge blurring. This is when the vector shape edge isn't sitting exactly on a pixel, and the edges
To sort this, you need to zoom in as far as the Zoom tool will allow, and then use the Direct selection tool to shift the edge until it's neat. A unified practice amongst my team is consistent spacing. We try to make sure all the elements on the page are spaced in increments of 5 or 10 pixels.
It's just so much easier to know that when you pick up someone else's file, all their elements will be spaced the same amount, saving you time measuring everything. You can use the Shift key when moving an object with the arrow key to space things out in increments of 10 pixels.
I'm not saying that you should always space stuff in 5 or 10 pixel increments, but if you do have a rule on a comp for how things are spaced, jot it down in a note so other designers will know what to look for.
Leave a trail
It's very useful to add your details to the File > File info menu. In all honesty, there are so many fields of data in this menu and I never use much of it, but simply putting your name in the author box is useful.
This way, another designer can find out who created the document. Consequently, you'll know who to chase should you have any questions, or who to shoot if they've flattened something you need unflattened!
I also like to make notes with the Annotation tool. Admittedly the annotation post-its can get annoying, but if you keep them off the document's editable area, they aren't so bad. If I get an idea while working on one layout I'll jot it down so I can come back to it later. I actually use real post-its to list out tasks I need to do for the day, but the digital versions are great for making sure others get all the information they need.
Include all options
Make sure you tie up all loose ends on your design. This will prevent the stressed-out developer asking you, "What happens when you rollover this?" Including rollover states, drop-downs, highlights and expanded menus in your comp gives those guys all they need to build the site. It's good practice to include several layer comps with all the different states of navigation.
There are a few things I tend to do a lot, so I create actions and droplets for them. This is pretty standard stuff, but something good to share. I always have a 'Save as jpg' action and droplet assigned to my F1 key. This action exports as a high quality JPG to a specified images folder on my desktop. I can then quickly batch a whole set of comps to get them ready to show to a client.