Rooted and/or custom ROM installed. Now what?
Get Set CPU. You can create profiles to change how your CPU acts. You can save battery this way.
Get AutoKiller. This is NOT a task killer. It clears EMPTY apps from memory. These are not apps that are running in the background or doing anything. Android does itself kill these, but this application allows you to change the thresholds of memory to do this.
Get Autostarts. Each android app listens to certain events that occur on your phone. Some apps listen to events that they do not require. This app allows you to prevent some apps listening to some events. For example, I don't want photo bucket to listen for an event based on my location. I don't need it to know where I am. TuneWiki does not need to know if I have installed an application.
Have you got enough space on internal memory? If not, you could flash a ROM with A2SD+ Support. FAQ here - http://androidforums.com/desire-all-things-root/220627-faq-apps2sd.html
What else can I do?
Well with custom ROMS, you may have the choice of installing a custom Kernel, which will run the CPU at the same speed as standard (998Mhz) but using less voltage (saving battery). This is called Undervolting.
At the same time, if you have SetCPU, you can overclock a kernel (if it permits) to run at a faster speed than normal, but at the same voltage as 998Mhz. You can compromise between the two.