Best Practices

What not to do:

• Don't use the administration account for anything other than setting up the machine and applications or changing "permanent" settings (if you want to, say, change network settings as a normal user you'll be prompted for the admin password, and since you'll do configuration changes less and less often as time progresses, this isn't a problem).
• Don't install "toy" applications as the admin user until you're sure you're going to use them. Most applications will run just fine from an Applications folder under your user directory.
• Don't open files or mail attachments that come from unknown sources. This is a basic security measure that is valid for any kind of machine, even for a Mac - no operating system can protect your machine from your mistakes... One way to improve your odds is to open Safari preferences and uncheck the Open "safe" files after downloading option. There is no such thing as a "safe" file, you have to be sure of what you download.
• Never, ever move Apple applications (such as Mail.app) to other folders. Mac OS X updates are not always very clever at updating built-in apps, so you'll end up with either two copies of your apps or a single broken one.
• If you're a Windows user: Don't try moving a folder on top of another with the same name - Mac OS X will replace the entire folder, i.e., it will not merge both folder's contents and will cause you to lose your data. This is a very significant difference between the Finder and Windows Explorer that catches most people at least once, and is simply a matter of different traditions (plus the UNIX underpinnings of Mac OS X, where things have worked like this from time immemorial). Drag the folder contents, not the folder itself.
• If you're a UNIX user: Don't fiddle with the system startup scripts unless you're sure you know what you're doing (which includes reading this and a bit more about launchd and the startup files themselves). Mac OS X doesn't use runlevels in the same way as "classic" UNIX systems, and things like network configuration, system services, and whatnot are not (necessarily) stored under /etc. You'll figure it out after a while, but don't go in thinking this is "just like any other UNIX system". It both is and isn't like other UNIXes, and like other UNIXes, fiddling under the hood is not to be done lightly until you've read the documentation.

Things You should do:

• Did you read the bit above about setting up your own user account? Go back and read it then.
• Disable Safari's "Open Safe Files After Downloading" preference. There is no such thing as a safe file anymore, and like e-mail attachments, you should only download and open files from trusted sources. Yes, I am repeating this again, just to make sure you get it.
• Tweak Safari preferences to enable tabs. You'll never go back.
• Create your own Applications folder inside your home directory (Mac OS X will change the icon accordingly) and try out new stuff in there. If it breaks, you won't break the machine for other people (and if a Mac OS X app breaks while running under an unprivileged account, it's badly written for sure).
• Put things you want to share among users in the /Users/Shared folder. You may have to tweak file and folder permissions a bit, but it's the easiest way (see my Address Book, iPhoto and iTunes pages for ways to share these applications' data).
• Consider using a separate user account for playing around (yes, another one, besides your own and the administration one). Fast User Switching makes it very easy to try out new software like this.
• Set up separate accounts for kids using the Simple Finder or Some Limits option under the Limitations tab.
• Use Exposé. Go on, press F9. Learn how to use F9, F10 and F11 to switch windows and drag-and drop files onto apps - there is nothing quite like it, and it's a real time-saver.
• Be mentally prepared to reinstall your Mac from scratch after a month or so of heavy fiddling. This is mostly applicable to the UNIXheads that like poking under the hood and will disregard every other warning so far - "Normal" users can disregard this hint, especially if you follow my previous hint and don't use the administration account for everyday work.
• Get as much RAM as you can possibly afford - 512MB used to be OK for general Mac OS X 10.4 (Tiger) use, but programmers or graphics-oriented folk will feel 1GB much more comfortable (I have felt little need to go beyond that so far, but G5 users that do a lot of video editing will most certainly disagree with me).
• If your Mac didn't come with a Mighty Mouse (as late 2005 models do), then get a proper mouse. I've said before that Mac OS X with a single button mouse is like rowing with a single oar, and I mean it: Mac OS X supports multiple-button mice, scroll wheels, etc. I use Microsoft wireless mice (the old kind, not the Bluetooth ones), which work perfectly and can be obtained in white (no sense getting a horrible blue and red mouse, now is there?).
• Although all new laptop models now have two-finger scrolling, people picking up a second-hand iBook or PowerBook should look at something like uControl or SideTrack to make better use of the trackpad.
• Drag Terminal.app to your dock as soon as possible (it's in Applications/Utilities). UNIX won't bite you, and some things are best done at a terminal.
• Use Microsoft's great Remote Desktop Client (recently updated to 1.0.3) to access your Windows XP boxes. VNC is good, but RDC is much faster and also allows you to transfer files across by mounting your Mac filesystem on Windows (it will appear as a network drive).