Moving from Nokia N8 to Galaxy S2 and then Galaxy Note, my enthusiasm of migrating to Android was bogged down by battery issues, I gradually got along with this fact assuming maybe Android phones are supposed to last only with 1 day battery back up max. Read lot of posts on battery saving tips, tried almost all of them and downloaded many battery saving apps, task killers etc from Google Play store, eventually it did not help much and as a heavy 3G and Wi-Fi user I started thinking that I may have to just stop bothering about the battery issues.
There was a stage where I started losing my Galaxy S2 battery within 8 hour span, even with heavy data usage S2 battery used to last between 10-12 hours, Android OS has many background processes running which constantly use phone memory and battery power. Onscreen widgets help you to keep yourselves constantly connected with news/social networking updates that you like to have on your phone offering easy accessibility, add live wallpapers and plethora of themes and launchers available for Android that offer enough eye candy to pep up your phone home screen.
If you also happen to download lot of apps out of curiosity to try them or for any specific requirement, they contribute further to the list of battery munching apps which would exhaust your phone battery charge at a higher rate, downloading battery saver or task killer apps would not help in the long run. Android apps with poor API (Application programming Interface) settings are major of battery draining issues, here are some basic steps that you can follow with the phone first:
1) Network Settings - Avoid keeping your phone on 3G/Wi-Fi mode while not using data, In my opinion it does not make much difference on voice calls with 3G network and while you are on the move being on the 2G network is your best bet to save battery drain resulting out of poor coverage in many places, try switching your phone to flight mode during night time.
2) Screen Time-Out and Brightness - Maintain 10%-20% screen brightness for regular indoor use, with a screen time out duration of 30 seconds, you can increase brightness while watching movies, games etc.
3) Manage phone profile: You would get basic option in most Android mobiles to manage phone profile settings however it takes lot of manual effort to customize and switch between these phone profiles every time, however there are apps both paid and free to help you manage these settings, let's take a look at few of them.
1) Setting Profiles (Paid and Free version): If you are looking to manage only your phone's profile settings, this app would be sufficient to offer ample customization options to configure temporary or automatic settings for managing sound, network, wi-fi, bluetooth, backlight or automate specific tasks.
Google Play link
2) Tasker(Paid): Tasker is one stop solution to manage actions and events on your phone beyond the profile setting and would require extensive input from the user which may feel like rocket science to begin with however it's like writing a visual script to manage your phone they way would think of it as a real smartphone.
Google Play Link
3) Onavo Extended (Free): This App helps you compress data usage while using your mobile internet service, which in turn helps you remain within the limits your data plan, What it actually does by compressing media and images goes almost unnoticed in your quality of browsing, you get to see report of how much data you have saved in form of Twitter/Facebook post count, email data usage stats etc.
Google Play Link
4) BetterBatteryStats(Paid): This app is a must have to save on android phone battery that would take some time for users to understand the real cause if your phone is getting discharged quiet quick, from the time you unplug your phone from charging, you can check 3 sections on this app.
b) Kernel Wakelock: Display's list of events again in minutes and seconds along with the count, however if you find any abnormal activity here, you can google the event name and you would find probable solutions on various android forums informing you how to deal with that particular process.
However this takes some searching with Trial and Error to fix battery draining issues that Google is not directly responsible for as phone manufacturers tweak Android OS versions with a taste of their proprietary interface UI (example: Samsung's Touchwiz)
I would also mention few details around Bloatware that comes with Android mobile phones and differs with each manufacturer. These are apps and related files associated with events that do not cause any harm if you happen to disable or totally uninstall them. Rooted devices have easy fix with Apps that can be downloaded from Google Store that automatically uninstall all these apps from your phone, however if you do not want to risk voiding warranty on your phone with Rooting, ICS now offers option to this the easy way as well.
Go to Settings --> Apps and click on the "All" tab which would show a list of applications and processes that you can disable and here's a link to a guide of apps and processes safe to be stopped or uninstalled mostly common to Samsung Galaxy S2 and Galaxy Note.
Please leave your comments if you also happen to know additional ways of improving battery life on Android phones, I'll give a try to few of them myself and quote those solutions left by you.