When TiVo hard drives get full, they begin erasing old shows that you might still want to watch. To fight this problem, add an external hard drive. TiVo will sell you an external drive for the TiVo HD and TiVo Series3; but by doing it yourself, you can get twice the storage for the same cost.
Note: This hack will void your warranty, and since the hardware connects online, you will probably get a frowny face written on your account. But TiVo doesn't go out of its way to punish users who try this hack. The process is time-consuming but not too difficult.
If you have an unmodified TiVo Series3, all you have to do is connect an eSATA drive. Power off the TiVo, plug everything in, and power up the DVR and the drive. To enable the drive, simply go to the TiVo's Settings, Remote, CableCARD and Devices, External Storage menu.
Many TiVo HD hackers suggest that you buy an A/V-marketed drive, which is designed for video performance, constant use, and (often) quieter operation. These typically cost a little more than standard PC hard drives, so consider whether that premium offsets possible lost shows and the time you might have to expend in replacing the drive again should it eventually fail. (I decided to use a 500GB drive I had on hand that isn't marketed for A/V use.) You'll also need a 3.5-inch eSATA drive enclosure, preferably one with a built-in fan and a power switch that stays locked in the on position.