Kiowa directed you to the most information about battery life. Thanks to him for that.
First, your battery will always drain fast after a fresh flash. I give mine a couple days before I decide if it is a good or bad ROM as far as battery use.
Second, Battery life depends on the reception and data connection that you use. If data is on all the time, the battery drains much faster and if you are in a poor or marginal coverage area your battery will drain faster.
Third, I always set my screen brightness to 60% rather than using the auto adjust which constantly increases and decreases the screen brightness. Not only do I find that annoying but I think it uses more battery power than just picking 60%.
Fourth, Weather updates and email push use a lot of data connection so I set them to check for weather updates every 6 hours and email every three hours.
Finally, - and some disagree with me on this - but the phone has circuitry that "learns" the battery charge/discharge rate and that gets reset whenever you flash a new ROM. It can take several days of discharging and recharging before it relearns the patterns of battery use on your phone. With my phone, I notice that after about a week of regular use, my battery levels tend to stabilize. I had run the same NRG ROM since January and I was getting just over two days of full use. I just flashed the last ROM (3/21) and I get one day at best...But, I know that will improve. It usually always does.
How you charge your phone also factors in as well. I will usually let mine drain down to about 25 or 30% before putting it on the charger. I also use a second battery in case I can't hook up to the charger in time. It helps to have a spare. And, as NRG mentioned, if your battery is old, it will lose its ability to charge over time and the amount of use you get from it will decline rapidly.
Also, this sounds somewhat counterintuitive but overclocking actually helps increase battery life. At least in my experience with it. I have been overclocking my phone to 748mHz and not only does it run quite well but it seems that whenever I overclock, my battery lasts longer. YMMV.
Hope that helps. But as Kiowa pointed out - and provided a link to - the WIKI has lots of very useful information.
David