Battery life has two dimensions: (1) how much charge the battery can hold, and (2) how this capacity changes over time.
In my own experience, I wore out a 13" PowerBook G4 battery in 13 months, bought a replacement, and saw it wear out again in about 14 months just by keeping the machine plugged in and sitting on my desk most of the time. After the 2nd replacement battery wore out (held less than an hours charge), I got more interested in what was going on and made some changes. The most visible change was keeping my laptop on a cooling stand.
When fully charged, Li-ion batteries have a limited shelf life and lose around 20% of their capacity per year. High charge levels and elevated temperatures hasten permanent capacity loss.
I was stunned to learn this, but it matches my own experience. The combination of normal laptop running temperatures (warm), and a fully charged Li-Ion battery was less than ideal. Contrary to popular wisdom, keeping Li-ion batteries fully charged as much as possible shortens their life.
Where did this popular wisdom come from? First, it is vital not to allow Lithium Ion batteries to completely discharge as this can create a safety hazard and will shut down the battery permanently. For this reason, the power system is designed around the battery used and shuts down when the charge becomes dangerously low. Second, deep discharge cycles are also known to hasten capacity loss. Since there is no "memory effect" with Li-Ion, waiting until your battery is nearly discharged is undesirable. For a typical portable device, this means it's best to charge the battery early and often.
But not all portable devices are used as portables anymore. A glaring omission from typical laptop systems is a feature to maintain the battery at less than fully charged to extend its useful life. My laptop easily spends 90% of its waking hours plugged in. I simply don't need the battery to be fully charged most of the time. As laptops have come to replace desktops for many users, the design of the battery system has failed to keep up with how it is often used. The ideal charge level for storing a Li-ion battery is around 40%.
One of the challenges Toyota faced in designing the Prius was battery longevity. Part of the solution they adopted was to maintain the battery between 40-60% charge as much as possible. The Prius uses nickel metal hydride batteries, but the principle is similar. Batteries last longer if you treat them gently.
You might be tempted to remove your laptop battery completely, but this is not a good idea. Modern laptops use the battery to smooth out spikes in the power consumption. If you remove the battery, the machine will respond by dropping the CPU speed to bring the entire system within the upper limit of the power supply.
Apple's latest designs use integrated Li-polymer batteries, so removal is impractical anyway. I don't know how Li-polymer batteries compare exactly, but the principle should still be valid. It would be helpful to make a distinction between charging for maintaining battery capacity (useful lifetime), and charging for the longest run time.