There are a couple of things one should avoid with Li-ion/polymer batteries as they can cause premature failure:
1) Exposure to extremes of temperature. For example try to avoid leaving it in the trunk if you are fortunate enough to live in an extremely hot climate, or unfortunate enough to live in an arctic climate.
2) Leaving it unused for long periods while fully charged. Something I've suffered from in the past with second batteries that I didn't use often. Hence I'm now just content with a sealed in battery given it lasts long enough for most occasions.
3) Draining every last ounce of juice from it.
None of these will guarantee premature failure, nor will avoiding them guarantee perfect battery life cycle.
Even if you discharge it every day, you're likely to get 3 years out of it, with approx 80% capacity left by the end.
Lets just say you molly coddle it, only discharging it once a month and holding onto the power cord like a baby grabbing their pacifier. The best possible outcome would be that after 3 years (come failure time) it still holds 100% (very unlikely). But what use is that to you, if you're never ever using that capacity anyway. The old saying "you can't have your cake and eat it" comes to mind. (Although in reality I suspect no matter what you do you can't have your cake, something else will eat it if you don't :/).
IMO if £109 (Apples prices) for a battery replacement worries you enough not to use the thing as a mobile computing device (kind of the point of a laptop), you're better off buying a cheaper computer (so you can more easily afford a new battery or computer in a few years time).
