Li-Ion batteries actually prefer to stay around mid charge.
The further you stray from that, the sooner they wear out. However, going to low is much, much worse. Letting it go actually dead, can kill the battery which is why it warns you so much as you get low. Batteries ship with a 50% charge for this reason, this is also why leaving them sit on a charger constantly is bad, they want to return to 50% and go dormant.
If you bought a generic Chinese battery, expect 1-2 years at most. OEM's typically last 25-50% longer, depending on how you treat it. Generic ones also usually have 10-20% less capacity given equal ratings and easier to kill by letting them go dead. OEM's can usually handle this a few times, depending on severity, generics can die the very first time they go dead.
Do NOT let the first charge go all the way down, that is an old Ni-cad thing. DO NOT do this on a li-ion, it was only done on ni-cads to prevent "memory". However, on the first charge, even if it says it's at 100% do not remove it, this applies to all batteries, give it a few extra hours. The last bit takes more effort and longer to fully charge, especially the first time, think of it as breaking it in. Li-Ion batteries go into a dormant state and it needs to wake up, not doing so will cripple the battery some as it will later refuse to access that last bit. This and letting it go dead are the two biggest things people do wrong with Li-ion batteries. Fully charge it the first time and don't let it go dead and it should last a long time.
Also, Li-Ion do not trickle.
Once they charge, the charging stops, I was surprised when I heard this, but plug your phone in before going to sleep and leave it plugged in as long as you can, you will actually see it start to discharge even as it sits on the charger. When it reaches a certain point it will begin charging again (some systems do, some don't, our phones do).
For those curious, nihm batteries were actually better in some ways as they don't have the memory issues of Nicads (they have it, just much less), don't die when they get low (dead is still death), and can trickle. The only real downsides to them was that they were much more subject to damage from being dropped and of course less capacity compared to li-ion, which is what ultimately did them in.
I also second the spare battery.
I have a plug in battery I got when I got the phone, it's designed to sit fully charged for long periods (works on anything usb), and I also bought two generic ones from China for $10 (Ebay). I highly recommend the latter. The plug in battery is nice because it sits a long time and works on anything, but the spare batteries are small, cheap and fit inside the phone. For $10 they were also half the price and came with a charger, I hardly use them now that we have better roms, but they were worth it.
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