[T-Mobile Samsung Galaxy S II SGH-T989] generalist:
Here are three techniques I use to save my phone's battery charge. They control the use of mobile data, cell radio, and Google services. In my experience they are extremely effective, but that's just with my phone as configured and my use patterns. YMMV.

I know there is precedent for some of these, at least in part, but I was unable to find them described either completely or (as with Google services) at all.

Depending on whether your phone is rooted, and on which ROM and kernel you run, and so on, whether or how you can do these things may vary. My SGH-T989 is rooted and is running

Android: 2.3.6

ROM: Juggernaut-v4.1

Baseband: T989UVKL1

Kernel: Bullet-1.8Ghz-v2.9b #60

I hope any or all of these three battery saving techniques are useful to you. None of them should be dangerous to try, but I cannot guarantee that. DISCLAIMER: If you try any of these techniques and something breaks or is degraded, I am in no way responsible. :)

I. Turn off mobile data AND turn off 3G/4G (2G/GSM only)

Turning off mobile data is probably the most well-known of these techniques. I have nothing new to say about it, but this: Turning off both mobile data and 3G/4G together radically reduces battery drain. I believe this is always true, but it is very noticeable when the phone is mostly idle for long periods (for example, when I wake in the morning after a full night's sleep.)

I believe that if the cell radio is on, then "2G/GSM only" is synonymous with "3G/4G off".

Bedtime is the obvious time to do this, but I plan to start doing it when driving, too. Mobile data and 3G/4G will be off, with the phone docked, but the dock not connected to power. Since I shouldn't be reading incoming email while driving anyway (*cough, cough*), I should have plenty of battery for calls and GPS if I start the drive with a fully charged battery.

The app Llama lets me make shortcuts for toggling mobile data and 3G individually, and enables their use either manually or event driven. (Luckily this feature works with my combination of phone and ROM; but the developer warns that it doesn't work with some phones and/or ROMs.)

II. Set Wi-Fi Calling to Wi-Fi preferred or Wi-Fi only

At home today, with IMS (Wi-Fi Calling) on constantly, no cell reception (I live in the boonies), and no calls yet, I had a hunch about this as I saw the battery draining too quickly. First I checked battery stats (Settings > About phone > Battery use) and saw that "Cell standby" was using a lot of battery. So I did the following:

1. Switched from "Cellular Preferred" to "Wi-Fi Preferred" in Settings > Wireless and network > Wi-Fi Calling Settings > Connection Preferences.

2. Rebooted into recovery, wiped battery stats (advanced > Wipe Battery Stats), then rebooted into the OS

3. Charged the battery to 100%, then disconnected the phone from the charger

4. Checked battery stats, now seeing that "Cell standby" was not using the battery. Success!

Step 1 is the key. The other steps were just to prove my hunch: with IMS on and with little or no cell signal, "Cellular Preferred" keeps the cell radio on, and "Wi-Fi Preferred" keeps it off.

While writing this post, just to be sure, I reverted to "Cellular Preferred", let the phone idle, and repeated the procedure. From seeing "Cell standby" eat battery, to successfully stopping it from doing so, I reproduced everything.

III. Turn off connectivity for Google Services apps

Since I want Google to know the least about me possible, I let my phone connect to their servers only as needed. With no cell reception at home, IMS is fine for calls, but is unreliable for SMS (I get messages, but the ones I send are often not really delivered.) So I use Google Voice for SMS.

By trial and error, I learned which Google apps need connectivity for Google Voice to send and receive SMS, and to get push notification of new SMS messages. They are:

Google Partner Services

Google Services Framework

Google Talk

Google Voice

I use the app DroidWall to toggle those apps' connectivity. Recently, I noticed that when they are connected (with IMS on, at least), those Google services apps use battery heavily. So now when I keep them disconnected, I know I am saving not only my privacy, but also my phone's battery.

You may need any of those apps to be connected for other reasons (e.g., synching data with Google.) A while ago, I found that Google Services Framework needs to be connected to make purchases through the Market app (now Play Store). Google Partner Services may also be needed for this, but I haven't tested it.