The first thing is root, because many essential tweaks require it. To root it, the bootloader must be unlocked first. I first took all the OTA updates to update it to the latest 6.0.1, then unlocked it and root it.
As always, I disabled everything I could on the phone, then used Debloater to disable these junk, mainly from carriers and google:
Processing changes to: com.google.android.backuptransport -> Status is now hidden
Processing changes to: com.google.android.feedback -> Status is now hidden
Processing changes to: com.google.android.onetimeinitializer -> Status is now hidden
Processing changes to: com.android.sprint.lifetimedata -> Status is now hidden
Processing changes to: com.android.managedprovisioning -> Status is now hidden
Processing changes to: com.google.android.setupwizard -> Status is now hidden
Processing changes to: com.android.sharedstoragebackup -> Status is now hidden
Processing changes to: com.google.android.marvin.talkback -> Status is now hidden
Processing changes to: com.vzw.apnservice -> Status is now hidden
Don't check the items remarked with "//" if you need to update Sprint PRL and data profile.
Double-tap to wake
Not only convenient, it also saves the power button. Just install this app and enable this fantastic feature. Now, how to put the phone into sleep without pushing the power button? Easy, install the Lock Screen app by WangQi. At this moment, you might want to disable the "Ambient display" crap.
Use a black wallpaper
It saves battery for an AMOLED screen.
Don't use the stock charger
With the charger that comes with this phone, charging is very fast - 15 minutes for 6 hours play. What you pay for that is heat that is always a very bad thing to electronic parts, especially the battery and screen. So, don't charge for more than 15 minutes or use a different charger. No, using a thin cable won't slow it down.
Reduce CPU speed
This phone has a really fast CPU, but it runs phone hot and hurts battery life. I use SetCPU to limit the max frequency to 1190 (other values might not actually work). Another big battery eater is the stupidly high resolution screen, the only thing we can do about it is to use night mode in Dolphin.
Enable notification LED
Simply install Light Flow (paid version) and follow this guide.
There are only stupid thing left is the ugly white Google search bar. I know I can use a 3rd party launcher but I don't want to for the obvious reason - Android is already a mess and I don't want to add even more like all the phone manufacturers do.
[update] Flashed to 5.1.1 (shamu-lmy48y) and got everything working fine. Then flashed to 6.0.1 (shamu-mmb29s). This time I did not use Debloater, only disabled some non-system apps that I don't use such as Chrome, Fit, Docs etc. So far it has been a smooth sailing.
Fighting nexus 6 for battery life
This is phone cases ought to be
Nexus 6 case mod