Have five-plus years on the original battery on our '97 850 wagon and it has yet to show any signs of problems. We live in Chicago, so it gets a winter workout. I've had similar battery longevities from most of my cars. I would attribute its longevity to a number of factors.

1) it has never been drained to a dead battery--this is just a matter of luck. From my experience, once you kill a battery, it has been damaged for life.
2) keep the battery scrupulously clean. Every time I wash my car, I clean off the battery, even if it is just to wipe the top with a damp rag. Batteries discharge through the path of dirt between their posts.
3) only use distilled water and check the levels frequently. I try to maintain a surgeon's mentality when topping-up the battery. When I remove the caps, I don't let any interior portion of the cap touch anything outside the battery. I try and make sure that nothing gets in to the battery but distilled water--no dirt, no dust, no contaminants.

Some of the best information on battery maintenance is in forums devoted to hobbies which involve automotive batteries. In particular, there is a forum at http://groups.yahoo.com/group/elec-trak/ for people devoted to electric tractors. Their eMail threads frequently discuss battery maintenance issues.