What's the best way to prolong the lifespan of a Macbook Pro battery? I have two Mac book Pros: 


A 2013 Unibody retina that I use every day
An older 2006 model with removable battery that I use a couple of times a month (when not in use, it's not connected to the AC. I
recently replaced the battery, as the old one swelled apart).


I use them as desktop replacements, so I don't use the battery that often. 

What's the best way to prolong battery lifespan? - If keeping the machines hooked up to the AC is a bad idea, then for how many hours each day should I use the battery?



Here is my research: There's a lot of conflicting advice. 

Calibration

A lot of sources, say you should calibrate the battery by letting it run down to 0% each month. For example, this Ask Different question, this Gigaohm article. All these articles were written in 2013/12 and cite Apple.com as the source for this advice.

However, the Apple.com page makes no reference to calibration. Is this because calibration is a bad idea, or is it because Apple no longer sells devices with removable batteries and so the advice is different?

Deep discharge

Other sources, such as this AskDifferent question, imply that letting the battery drain is a bad idea, so using the AC whenever possible is actually a good thing. 

Please note I am talking about the overall battery life span (e.g. the time the battery has before it needs to be replaced) and not increasing the day-to-day hours of the battery.
What's the best way?

Don't worry about it and just use your MacBook Pros for what you bought them for.

Why?

Physics.  No matter what you do, you can't change the laws of physics and lithium batteries are governed by the number of cycles.  A complete discharge and recharge is one cycle.  A 20% discharge 5 times is one cycle, a 5% discharge 20 types is one cycle. 

Whether that cycle takes a half day, a full day or a week all depends on how you use it; but I am guessing you didn't buy a MBP to sit on a shelf all day long doing nothing.  

Even when you keep your MBP plugged in, you are reducing the per day cycle count; however you are not eliminating it.  But the bigger question is, did you buy a MBP to sit at a desk all day?  If so, you should have bought an iMac instead - there's no battery so you eliminate this problem altogether.

That said, there are things you can avoid that will shorten the life of your battery and those things include:


Extreme temperatures - high heat and freezing temps can kill a battery prematurely.
Drops and physical stress can damage the battery shortening its life
Not using the right charger can harm your battery.  Always use a GENUINE power adapter, not some discount charger you find on eBay.  


When they do die, it's always best to get a good quality replacement battery from a reputable manufacturer.  The 2006 MBP uses an external battery so it's fairly easy to find replacements - and some even come with generous warranties.  

The 2013 MBP Retina batteries require the back cover to be removed, but are actually pretty easy to replace.  You can find many good replacements now on the aftermarket, with generous warranties as well.  

When you get down to it, the battery prices are not that all off putting.

The bottom line is that you batteries have a life span that's governed by use and there is nothing you can do short of not using your MacBook to change that.  But...that kind of defeats the purpose of having the MacBook in the first place.