The HTC desire is a nice phone, but the battery life sucks. Short of getting a bigger battery (which you can do, but costs £££) there are a few things you can do to extend your battery life a bit.

If you go into the menu, settings, about phone, battery use, you'll see the main things that are draining your battery such as:

 Now the screen is probably the biggest drain. Switch it off via top button whenever you put the phone down. Another huge drain is signal. If you're full bars no problem, if you phone is constantly searching for a signal this will drain your battery fast. It's a good idea to have a few widgets added to your phone (add via long-press/widgets/settings):

If you're indoors, at home or work where you have access to wifi, switch off the mobile (3G) signal via the widget. You will be surprised just how much battery that saves alone. Secondly, when you go out, switch OFF the wifi, and switch on the 3G. Walking around with your phone constantly searching for wifi hotspots isn't doing your battery any favours either. (may seem a chore, but pressing two buttons when you enter/leave takes seconds and becomes habit).

Also, turn off Bluetooth unless you need it, in the car or whatever, then you can switch it on with one press too.

Make sure GPS isn't constantly running too. It's one thing GPS switching on if you're running Google Maps, but if there is an app that is using constant GPS then you want to switch that puppy off, or uninstall it.

Another thing, make sure you don't have seven screens full of widgets - each of them is an app running and draining your battery. Have your clock, a few shortcuts, and your power widgets, and learn to live without the rest.

Doing these simple things gives me 18 hours of battery time (ie charge overnight EVERY NIGHT), and you should last the day. (that's using Tweetdeck too). However, if you're out for the day taking photos, I would put a spare power lead in your bag, as battery will dissapear fast. Again, it depends on how much you use it, and what apps you are using.

Best thing I ever did was root my Desire, and put the Insertcoin ROM onto it. It has better hibernation, so that when your phone screen is switched off it uses practically no power, and has extended my battery to almost 24 hours before my phone screams for juice. 

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Update: I charged my Desire at 7am this morning, and it's now 22:30pm with 66% battery left. Not too shabby. For the Desire, anyway