You may have watched my short video on how to get longer battery life on your HTC Desire , and in it I say the best way to extend battery life is to turn off the internet connection when you're not using it.
In the video I say that the way to to this is by pressing the power button on the phone, then the "Mobile Network" option to toggle the internet on and off - but there is a quicker way.
"bspiner" on Youtube pointed out that there is a widget to do this which you can add to any of your home screens. Simply press the "plus" button, "widget", look for the "settings" widget, and press the arrow to the right of it. Choose the "Mobile Network" widget, and you'll have an icon with which you can simply flick your internet on and off without having to use the power button.
Thanks to bspinner for the suggestion.