Hi,
There's no definitive way to maintain your laptop battery, using laptop connected with AC adapter will not damge the battery, if you recharge the battery often, it will reduce the usage time, you can monitor the battery life by installing HWMonitor and keep monitoring the wear level, it all depends on how you use your laptop, I use it on a desk most of the time, so I keep connected to AC most of the time.
You can adjust the settings for Critical battery level and Reserve battery level as you prefer(snip) in Advanced Power options, do not select 0% as it will drain the battery completely and it's not good for battery health.
Yes, you can keep connected to AC even after 100% charged, if you disconnect it to move away from your desk, you can recharge any time you want to and do not have to wait for 20% or any set level.
http://www.cpuid.com/softwares/hwmonitor.html
