The battery drain on your Eris can certainly be mitigated but as dcfan said, since it is a smart phone, it is going to be a hog no matter what you do.
Another place you can look to increase battery life instead of what programs are running in background is how often your apps such as facebook, twitter, weather, news, mail, flickr, etc sync and set them to long periods, that is sync once a day as opposed to every hour. Go to settings>accounts&sync and tweak the settings there (for fb, flickr, google, twitter and weather). Some apps you have to set the sync time in the app settings.
You can turn the sync off completely or set it to do it manually in some apps. Some people turn off everything, apps sync, gps, wifi, screen brightness to a minimum. I have pretty much given into the fact that the battery isn't going to last a day unless I charge it a bit because I choose not to turn off everything and because I like to check my fb, email, twitter, etc. 30 times a day....that is what I bought a smart phone for. So i bought a cheap charger for my work desk and car (although I only use this for trips) and I usually don't have a problem. You can also buy a bigger battery if you want but that changes the footprint of the phone. It just depends on what you want out of the phone. Just give it some time, play around with it and check out the forums if you need help. They have been indispensable to me.
Also, I suppose you know you can turn off gps and wifi when you aren't using them to save battery life.