Buying Additional Storage
Through most of this article, I've talked about ways to make do with limited storage space. However, as in life, there's a limit to how far you can stretch your storage space and how much stuff you can fit into it. If your closets no longer accept anything new, you may need to rent a storage unit or find a bigger home. With your Mac, you may need to add more drive space to meet your needs.
The good news is that plenty of options are available. If you have a desktop Mac (iMac, Mac mini, or Mac Pro), you can simply purchase additional hard drives. In the case of a Mac Pro, you can get additional internal hard drives; for other Macs, you'll need external USB or FireWire drives. As the overall need for storage continues to grow, so do the capacities for new hard drives, while the prices tend to get lower per gigabyte every year.
But what if you work primarily on a Mac notebook? One option is to add an external drive, keeping only the files you need at all times on the internal hard drive (a particular issue with the MacBook Air), or purchase a portable drive such as one that draws power from the Mac's USB or FireWire ports. Or you can replace the internal hard drive with a larger one.
Beyond adding a larger hard drive (internal or external), you can try using a network-attached storage (NAS) device. These devices are essentially hard drives or hard drive enclosures that attach to your home or office network and can be accessed using wired Ethernet or Wi-Fi by one or more computers as additional storage space. They're a good choice not only for adding storage, but for sharing files across multiple computers.
There are many such devices on the market, but Mac users may want to look at Apple's Time Capsule, which is an integrated Wi-Fi access point with a built-in hard drive that can be shared with Macs and PCs and function as a Time Machine backup drive for one or more Macs. Similarly, you can attach a USB hard drive to Apple's AirPort Extreme Base Station for shared network storage. Both of these products can also support sharing a USB printer with Macs and PCs.