In an ideal world, we don't need to worry about power consumption. Compute devices consume the minimum power it needs to get things done. When a piece of hardware isn't used, it simply doesn't consume any power at all. Technology is improving all the time. Maybe someday we'll reach that ideal, or close enough to it.
Right now though, we still need to do some of the leg work. True, smartphones and the OS that run them should take care of this for us. In many ways, it is doing just that. It's just some cases can't be dealt with effectively yet. That's where the human being needs to come into play.
I've only owned my Desire HD for scant weeks, so I'm very much still learning the ropes. Here's some battery saving settings I've done to help maximise battery life on my smartphone. I've followed a minimalist philosophy when deciding for or against various features available in smartphones.
Enable wi-fi and/or Bluetooth as needed - Wi-fi / Bluetooth consume power even when not used. It's not like I'll be within range of a wi-fi network all the time, and I definitely don't use Bluetooth all the time. If I need it, it's just a few taps away so I turn it off until I need it to conserve precious battery power.
Manually set screen brightness - The Desire HD uses a LCD screen which reduces my options but that doesn't mean there's nothing that can be done about screen power consumption. I find the screen really, really bright even with auto brightness, so I've disabled it and manually set brightness to a lower level. It's still bright enough to see comfortably even outdoors but it's much less bright than the default settings. I've also reduced screen timeout from one minute to half a minute. Most of the time that's enough.
Turn off vibrate - I actually find vibrate quite annoying, but I used to use it with my Nokia 5220 since it alerted me to calls and messages especially in noisy places. Later I found I started getting sensations of the phone vibrating even when there's no call or sms. I seem to be developing ringxiety. So I stopped using the vibrate and I'm keeping that habit with my smartphone.
GSM only - The Desire HD supports both GSM and WCDMA networks, and it's set to auto detect the correct network to use. My service provider uses GSM so I manually set it to GSM. No need for the phone to waste time and power figuring it out for me when I already know.
Turn off 3G Data - I don't have a data plan. I use wi-fi at home. Outside I use wi-fi if it's there, else I just don't use mobile Internet at all. This saves both my battery and my wallet.
Turn off auto-sync - Like I said, I don't have a data plan so I use wi-fi. Even connected through wi-fi there's no need for me to get the latest news and status updates. I'll manually refresh as I need to. This way I can focus on doing instead of keeping up.
Tweak sound settings - I fail to see the point of hearing sounds when I tap on an app or type on the on-screen keyboard, so I disable sound notifications. I also adjust volume so it's just nice: not loud but comfortable enough to hear.
As you can see, it's nothing that's not been done before, but we all need a little reminder every once in a while that the little things can add up to a lot if we take them for granted. One reference I've used is the recommendations from Apple for the iPhone. Some might find that a surprising source of information, but an iPhone is also a smartphone and all smartphones will have many features in common even when running on different OSes.
Android comes with a lot of settings that can be tweaked to your heart's content so it takes some exploring to find all of them. There's also apps out there to consolidate these for you. Do you have more to share? Right now, I'm getting about 2 days worth of use on a single charge, but I believe there's more that can be done to improve battery life further. I'd love to know more.