It maximize battery longevity you power down the device and charge to 80%. I honestly have never turned off any phone to charge.
Also I try to recalibrate every 40 days. Since the iPhone has a none removable battery I would try to take care of the battery. Anything over 80% is stressful to the battery.
Hmm. Well, after 3 years of being continuously active, my iPhone 4 still holds a charge fine. Lasts throughout the day, even on iOS 7, no problem. I also didn't let it run down to completely dead very often. As I said, maybe a handful of times in 3 years. A far cry from the "once a month" recommendation. It also was routinely charged until the charging indicator changed from the lightning bolt to the plug symbol, indicating a topped off full charge. I did not, however, leave it charging overnight... unless I fell asleep with it on the charger... in which case, it was maybe on for 4 hours tops, as I wake up throughout the night.
I recently sold it to my friend's wife after picking up my 5S, and she has no complaints of any battery life issues.
Knowing these tips and tricks
now, I may treat the battery in the 5S differently... but the ol'iPhone 4 was, and still is, a trooper... considering my less than optimal charging behavior.