I've had my Desire since Nov and initially found the battery to last no more than a day. I wasn't disappointed per se since I had read the same as you, however, I now get a minimum of 2 days out of the battery, sometimes 3 days or more.
As cazzz stated, disabling active wallpaper is a good idea but the best thing you can do is put the HTC mobile data toggle button onto one of your homescreens. By disabling mobile data (3G data) when you're not using it you save a massive amount of battery life. Use the settings on the phone to turn off automatic application syncing and add the HTC button to manually sync selected programs with a single press. Connecting and disconnecting from your mobile data network takes seconds and doesn't interfere with the phone signal itself.
Also, don't use a task killer. Despite what some people will tell you, Android manages tasks fine on it's own. Tasks that are not actively being used whilst still in memory do not have an impact on the battery.