Keep all radios you're not using (GPS, wifi, Bluetooth, mobile data) turned off when you're not using them.
Keep the screen brightness as low as you're comfortable with.
If you root, use
Wakelock Detector (if you don't root, read
Wakelock with non-rooted KitKat first) to see what's keeping the phone awake and
Greenify to stop it. (BTW, root is the Android equivalent of jailbreak.) Rooting itself - which is just adding a file (su) to the filesystem - doesn't do anything as far as battery usage. And you don't have to root to flash a new ROM. You root the ROM, not the phone. (The su file is added to the ROM.) So if you root a phone, then flash a new (and almost always rooted) ROM, you wasted the time it took to root the first ROM.
Kernels? If the kernel can change the clock speed (CPU clock, not time clock), which just about ll phones do now, changing the kernel isn't going to buy you anything. Changing the kernel in older phones just allowed you to run the CPU slower to save battery (which made the phone run slower). An app that can handle clock speed change better may save a little battery.
Keep your email sync set to Push or Automatic (the server sends the email when it gets it) rather than a fixed polling interval, unless you get more than 1 email every 15 minutes. Your email account(s) will have to be set to IMAP, rather than POP3. (Or, if you don't care to be notified about new emails, and just want to see them when you go to look for them, set it to manual.)
There are/may be/may be in the future, ROMs done to save battery (at the cost of speed and/or features - you can't get all 3). But rooting and/or flashing a ROM, except with HTC, voids the warranty (it's the manufacturer's decision, not something technical), so you have to decide.
(BTW, don't start worrying if you notice that RAM is always full - it's supposed to be in Android. Read
Multitasking the Android Way.)