I wouldn't be too strict with this "do not let it fall below xy%" rules. Might be good to keep it in mind, but not as a strict rule. Because if you "never let it fall below 50%" you effectively cut down your capacity by 50% on day one.
The usable capacity of a lithium battery
is 40% to 50% of the capacity rating. Discharging it past 50% shortens the life. Down to 40% doesn't shorten it much. Down to 20%? You might get 2 years out of it, if you're willing to use a battery that can't hold much charge for a few of its last months.
Charging at 50% can give you a lot more than the spec says - I have 10 year old batteries that still give me about 90% capacity - and were charged around the 50% point all their lives. (Remember, only 90% of the gas you pay for is gas - in most places, in the US, at least, 10% is Ethanol, which doesn't give you much mileage. Produce sells by the pound, and you throw out Bell pepper seeds and core. There are a lot of things we can't use 100% of - lithium batteries are one of them. [And I'm not going to carry a GelCell in my pocket to power my phone, just so I can discharge it flat.])
Leaving the charger plugged in all day - or all week? The only problem is that if something goes wrong with the charging circuit, the house will probably burn down after the explosion. But if the phone works? You can leave the phone on the charger for months. Overcharging a lithium battery today is close to impossible.
A few battery killers:
Deep discharge.
Use when the battery is very cold (like sitting in a car overnight in sub-zero weather).
Overheating.
And one we can't do anything about - dendrites. Once you get one, it's time to find a battery recycling bin.
Darth Spock's link is to a page on Android central - which links to Battery University. And those people know about batteries, like surgeons know about bodies - they take batteries apart, torture them to death, test them under any and all conditions. It pays to take a day and read their course.