If you buy a new car and never change the oil, it will eventually be ruined. Computers need maintenance too. But you have to be the mechanic.

Free Updates

Applications

When applications (e.g. Firefox, Word, Photoshop) pop up an "updater" box for a minor (free) upgrade, accept it: most of these are security fixes or fix crashing problems.

If you use anti-virus software, you must keep it and its virus definitions up to date.

Apple

When Apple Software Update wants to update your software, these updates often fix crashing bugs or security problems. Often, these updates are only provided for the most current version of the OS, so that's a good reason to stay up to date. (Dismount (eject) and unplug your external drive before applying an OS update. Plug it in again after the update has finished.)

Sometimes, for updates of the operating system, it is a good idea to wait a few days and watch the comments in MacInTouch to see if others are having trouble; this kind of trouble may affect older computers more than recent ones. I usually wait a week or so before installing OS updates, and watch MacInTouch for complaints. When I do install, I make sure each computer is fully backed up, and then run Disk Utility to repair permissions on the disk. I then eject any external disk drives and disconnect devices other than the keyboard and mouse. Some users have found that it is safer to download the "combo updater" and apply that, rather than just saying "install" to Software Update.

• (01 Dec 2010) I tried update 10.6.5 on a Mac Mini and had problems, so I went back to 10.6.2 on that machine.
• (12 May 2011) There were reports in MacInTouch and other web sites that MacOS update 10.6.7 introduced a serious problem with fonts. I held off installing the update, even though that meant I couldn't install other security fixes. After about 6 weeks, Apple released the 10.6.7 Font Update and users reported that this change fixed the problem. I installed it, and then the Font Update patch and security update. No problems so far.
• (02 Feb 2012) Apple released Security Update 2012-01 for OSX 10.6.8. Many users reported to MacInTouch that installing the update broke all Rosetta emulation for PowerPC code. A few days later, Apple released a revised version of the update that fixed the problem.

Paid upgrades

Applications

Some applications (e.g. Adobe Photoshop) release major new versions every year or so. Usually these upgrades are some fraction of the cost of the new application. Sometimes you don't need to buy them: they add features you can do without. I have been buying about every other new version of Photoshop and Illustrator.

Apple

Every few years, Apple will come out with a new major version of OS X, usually a paid upgrade for about $100. Don't rush into upgrading: often it is wise to wait for the .1 or .2 version. But do upgrade eventually; Apple eventually stops supporting old versions with security patches. Also, if your OS is too old, you will eventually be in the situation where you can't install some new program. When you upgrade the major OS version, sometimes this means that you have to upgrade some applications as well.

I held off on switching from 10.4 "Tiger" to 10.5 "Leopard" for almost a year, because I was using a programming tool (Eclipse) that wasn't ready for 10.5. Finally I was able to make the change and I'm glad I did: Leopard was faster, had nicer features, and supported  Time Machine. Mac OS 10.6, "Snow Leopard," was available for $30 in fall of 2009; I waited almost a year to install it.