A
A few leftover files from an application you deleted won't have any effect. Neither will login items that refer to applications you've uninstalled. Just delete them.
B
Links have been posted in this thread to the "macupdate" website. Do not follow the links, and never download anything from that site. It intentionally distributes OS X malware by packaging some free applications (such as "Firefox" and "Skype") in an unnecessary and malicious "installer."
All software should be downloaded directly from the developer's website or from the App Store. Don't trust any site such as "macupdate" that aggregates links.
C
How to maintain a Mac
Make two or more backups of all your files
One backup is not enough to be safe. A copy of a backup doesn't count as another backup; all backups must be made directly from the original data.
Keep at least one backup off site at all times in case of disaster. Backing up to a cloud-data service is one way to meet this need, but don't rely exclusively on such backups.
In fact, don’t rely on any single backup method, such as Time Machine.
If you get an indication that a backup has failed, don't ignore it.
Keep your software up to date
In the App Store or Software Update preference pane (depending on the OS version), you can configure automatic notifications of updates to OS X and other Mac App Store products. Some third-party applications from other sources have a similar feature, if you don’t mind letting them phone home. Otherwise you have to check yourself on a regular basis.
Keeping up to date is especially important for complex software that modifies the operating system, such as device drivers. Don't install such modifications unless they're absolutely necessary. Remove them when they are no longer needed. Before installing any Apple update, you must check that all system modifications that you use are compatible. Incompatibility with third-party software is by far the most common cause of difficulties with system updates.
Don't install crapware
...such as “themes,” "haxies," “add-ons,” “toolbars,” “enhancers," “optimizers,” “accelerators,” "boosters," “extenders,” “cleaners,” "doctors," "tune-ups," “defragmenters,” “firewalls,” "barriers," “guardians,” “defenders,” “protectors,” most “plugins,” "virus scanners,” "disk tools," or "utilities." With very few exceptions, such stuff is useless or worse than useless. Above all, avoid any software that purports to change the look and feel of the user interface.
It's not much of an exaggeration to say that the whole "utility" software industry for the Mac is a fraud on consumers. The most extreme examples are the "CleanMyMac," "TuneUpMyMac," and “MacKeeper” scams, but there are many others.
As a rule, you should avoid software that changes the way other software works. Plugins for Photoshop and similar programs are an obvious exception to this rule. The Mac App Store has extensions for the Photos app, and perhaps others, that should be safe. Safari extensions, and perhaps the equivalent for other web browsers, are a partial exception. Most are safe, and they're easy to get rid of if they don't work. Some may cause the browser to crash or otherwise malfunction. Some are malicious. Use with caution, and install only well-known extensions from relatively trustworthy sources, such as the Safari Extensions Gallery.
Only install software that directly enables you to do something that you need the computer for. For example, a word processor enables you to write. A video editor enables you to make movies. A game enables you to have fun. But a "cache cleaner" or a "malware scanner" doesn't enable you to do anything. You didn't buy a computer so you could clean caches or scan for malware. Just say no to anyone who wants you to do such things.
Never install any third-party software unless you know how to uninstall it. Otherwise you may create problems that are very hard to solve. Do not rely on "utilities" such as "AppCleaner" and the like that purport to remove software.
Don't install bad, conflicting, or unnecessary fonts
Whenever you install new fonts, use the validation feature of the built-in Font Book application to make sure the fonts aren't defective and don't conflict with each other or with others that you already have. See the built-in help and this support article for instructions.
Deactivate or remove fonts that you don't really need to speed up application launching.
Avoid malware
"Malware" is malicious software that circulates on the Internet. That kind of attack on OS X was once so rare that it was hardly a concern, but it's now increasingly common and dangerous.
There is some built-in protection against malware, but you can’t rely on it—the attackers are always at least a day ahead of the defense. You can’t rely on third-party "anti-virus" or "anti-malware" products for protection either. The only safety lies in safe computing practices. The "anti-virus" industry will always try to persuade you that you're helpless to protect yourself from malware attack unless you use its products. You're not helpless—rather, that industry is helpless to protect you, just as it's helpless to protect Windows and Android users, whose platforms are infested with malware despite the fact that they spend $75 billion a year on "anti-virus" software.
Never install software from an untrustworthy or unknown source. If in doubt, do some research.
Any website that prompts you to install a “codec” or “plugin” that comes from the same site, or an unknown site, is untrustworthy.
A stranger on a website who is strangely eager for you to run a free application, and won't take no for an answer, is untrustworthy. What's in it for him?
Software that doesn't come from the Mac App Store, such as Adobe Flash Player, must come directly from the developer's website. No intermediary is acceptable, and don’t trust links unless you know how to parse them.
Any file that is automatically downloaded from the web, without your having requested it, should go straight into the Trash.
A web page that tells you that your computer has a “virus,” or that anything else is wrong with it, is a scam.
In OS X 10.7.5 or later, downloaded applications and Installer packages that have not been digitally signed by a developer registered with Apple are blocked from loading by default. The block can be overridden, but think carefully before you do so.
Because of recurring security issues in Java, it’s best to disable it in your web browsers, if it’s installed. Few websites have Java content nowadays, so you won’t be missing much. This action is mandatory if you’re running any version of OS X older than 10.6.8 with the latest Java update. Note: Java has nothing to do with JavaScript, despite the similar names. Don't install Java unless you're sure you need it. Most people don't.
Don't fill up your disk or SSD
A common mistake is adding more and more large files to your home folder until you start to get warnings that you're out of space, which may be followed in short order by a startup failure. This is more prone to happen on the newer Macs that come with an internal SSD instead of the traditional hard drive. The drive can be very nearly full before you become aware of the problem.
While it's not true that you should or must keep any particular percentage of space free, you should monitor your storage use and make sure you're not in immediate danger of using it up. According to Apple documentation, you need at least 9 GB of free space on the startup volume for normal operation.
If storage space is running low, use a tool such as OmniDiskSweeperto explore the volume and find out what's taking up the most space. Move seldom-used large files to secondary storage.
Relax, don’t do it
Besides the above, no routine maintenance is necessary or beneficial for the vast majority of users; specifically not “cleaning caches,” “zapping the PRAM,” "resetting the SMC," “rebuilding the directory,” "defragmenting the drive," “running periodic scripts,” “dumping logs,” "deleting temp files," “scanning for viruses,” "purging memory," "checking for bad blocks," "testing the hardware," or “repairing permissions.” Such measures are either completely pointless or are useful only for solving problems, not for prevention.
To use a Mac effectively, you have to free yourself from the Windows mindset that every computer needs regular downtime for maintenance such as "defragging" and "registry cleaning." Those concepts do not apply to the Mac platform.
A well-designed computing device is not something you should have to think about much. It should be an almost transparent medium through which you communicate, work, and play. If you want a machine that needs a lot of attention just to keep going, use a PC, or collect antique cars.
The very height of futility is running an expensive third-party application called “Disk Warrior” when nothing is wrong, or even when something is wrong and you have backups, which you must have. Disk Warrior is a data-salvage tool, not a maintenance tool, and you will never need it if your backups are adequate. Don’t waste money on it or anything like it.
