I started out with Advanced Task Killer, which Verizonwireless had already put on my phone. It worked really well, although I didn't always know which tasks I could "kill" without affecting the operating system. After a short while it occurred to me that it would be a lot easier if this was automatic. So I looked for and found an app to do just that, Automatic Task Killer. This seemed great it automatically did what Advanced Task killer did. Sometime later I started getting an error on my device. I couldn't figure out where it was coming from and wasn't sure of the best way to go about finding out. I spoke to VerizonWireless Support, they eventually bounced me to HTC support. I was told it was probably some application I downloaded and to reset my device and then reload each application until I started getting the error again. Luckily it was an "A" application - Automatic Task Killer. So I got rid of that and went back to using Advanced task killer.
I eventually tried Juice Defender, it seemed like a good idea, but the log from JuiceDefender showed it was turning on my device at night. I may not have had it configured correctly, but I decided to uninstall it. The next app I tried was Power Manager Full. I tried the free version first and it looked really promising. This app comes with predefined profiles that kick in under predefined circumstances (when plugged in turn on wifi and brighten screen, for instance). The paid version lets you define your own senarios. I liked it a lot and went ahead and bought it. After some time using it I found it was taking up more battery than it was saving, so I uninstalled it (your mileage may vary, this was my experience on a non-rooted phone).
I finally got fed up with trying to extend the life of the HTC battery. If I used absolutely nothing, I could make it last for about 15 hours, but what's the use of having the phone if you aren't going to use it?! So I bought a better battery with the same exact footprint so it fit in the case. It was like going from a gas guzzler to a fuel efficient car, my battery now lasts 12 hours with regular usage. The battery I got was a Seidio. I recommend it highly.