1) Don't install any task killing apps. Anyone who tells you otherwise simply does not know what they are talking about.
2) Avoid any "charge it for an hour, unplug for 10 mins, charge for an hour" nonsense. These battery "fixes" are superstitious twaddle.
3) Take charge of your apps and widgets. Sync manually. Removed battery hogging widgets. Turn off 3G and wi-fi when you don't need them.
4) Don't use live wallpapers and turn down the screen brightness.
5) Monitor your usage using *#*#4636#*#* and identify any apps that may be sapping power.
Why is it that everyone has a different opinion?
1) Yet task killing is needed sometimes for "stuck apps" which drain battery life - I now use a program that monitors memory usage and lets me kill apps that should have shut down after I quit them. Saying there's no need is going too far. Saying you don't need to constantly use it, is more accurate.
2) This is one where other people have said it works and no-one seems to agree on this. I haven't tested it so I don't know but charging batteries is not an exact science because it's a chemical reaction after all.
3) This one I partially disagree with. Manual syncing defeats the point of a phone. If I only discover I received an urgent email after an hour or more, it would be too late. Sure, for people who don't read emails on their phone, it's not a big deal. But syncing of news, facebook, etc. can be a nuisance when turned off because it takes 10-20 secs to manually sync so a quick look would turn into a lengthy look.
Another thing: the problem with disabling 3G and Wi-fi, is that people all too often forget to turn them back on which can have painful results (like blasting through your data package or crossing it which would result in a lengthy bill).
Generally, the best advice I can give is to find a way of working that requires as little fiddling and adjusting as possible. That means leaving everything on if you might need it. 3G and Wi-fi both on won't drain the battery that much at all. Unless you know you'll spend a few days at a place with Wi-Fi, go ahead and leave 3G on but when on the move, its beyond ridiculous to keep switching stuff on and off. It's not a pleasant way of working either. The less "attention" your phone needs, the better.
4) Live wallpaper only drains the battery when it's visible i.e. when not in an app or when the phone is in standby mood I was told. how often do you leave your phone on the front page? Turning it off might make a minute difference but the result would be losing a cool and attractive wallpaper. Reminds me of people saying to turn off Aero in Vista for the tiny performance boost - is it really worth it?
Screen brightness is something that generally won't make that big a difference anymore. Again, you're sacrificing the attractive clarity and colourfulness of your phone for maybe a few % of power saved. If I were to watch a movie with brightness turned lower, it would harm the enjoyment.
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In the end, most of these measures are a big hassle compared to the result. It's way easier to just get another battery and replace it if you really fall without power.
Me? I watched a 2 hour HD movie yesterday, spent an hour surfing, 5 minutes calling, installing apps, using Dropbox extensively, using my phone as shopping list, an hour of listening to music, an hour surfing at the very least ... and I had 50+% left. This was from 11am till 11pm (I leave it plugged in at home when I don't need it). This is WITHOUT any of the measures above. Syncing is left on, weather is left on, Wifi and 3G is left on, etc.