A partial discharge reduces stress and prolongs battery life. Elevated temperature and high currents also affect cycle life.
100% DoD 300–500
50% DoD 1,200–1,500
25% DoD 2,000–2,500
10% DoD 3,750–4,700
And temperature affects the life as well
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries
These seem to be best practices, but are there any practical methods of preserving and prolonging the battery life of a power bank?
perhaps simply keep your power banks plugged in is the best way to go 
NEVER over-discharge it.
store around 80% charged if not used for a long time.
@Kyle_Hu Wouldn’t recommend always having them plugged in, found that to be a guaranteed method of reducing lifespan away from the power sockets
Just like laptop and phones, Anker power banks has over-charge prevention, so there’s no need to worry about plugged-in long term.
If the li-ion battery self-discharges over-time, and exceeds certain threshold, it gets topped-off.
Perhaps use it once a month is a good idea.
@Kyle_Hu They might have overcharge protection but in the 1000+ laptops we have on my site I’ve learned to not fully buy into the ‘overcharge protection’, they will degrade with constant charge
I believe Anker recommend a full recharge cycle every 3-4 months depending on the model you have. I would stick to there recommendation to enforce your warranties…
Generally you want to keep them between 40-80% for long periods of inactivity. If you are using them a lot most batteries have about a 500 cycle life.
that’s why Prius’s traction battery lasts forever 
i can’t argue with that