Really you don't. Android is set up to manage it for you. When you exit an app it's basically saved in it's current state and then closed out completely as it is no longer needed. Taskiller has color coding that shows if the app is taking up memory, running as a service, or as an active app.
The problem is developers. Some of them have their apps stop running altogether, some switch to a service, some have an exit button, some do not. The problem is not the OS, as it was built to natively manage your memory. The problem is that some developers aren't taking that into account. They develop apps that consume unnecessary battery, memory and resources. The smart ones are learning how to curtail this, the stupid ones just don't care.
The other problem is users. Some people want every app out there and don't look at what the app does. Some keep running in the background no matter what you do, bad app, bad developer, uninstall.
iPhone users carry a charger. The "Mid-day Charge" is something they got used to as phone owners. They just didn't complain enough about it for anyone outside of the mac echo chamber to hear about it. Google Mid-Day Charge and you'll find that it's a very real apple issue.
Battery Management issues:
Download Locale and have it turn off things you're not using, turn down the screen, change the screen timeout and get the plugin to turn off auto-sync when you don't need it.
If you know you're going to be at work and busy for more than 2 hours, turn your phone off. That will save battery.
Biggest battery drainers:
Screen Brightness
GPS
Bluetooth
AutoSync
Widgets
Hopefully seidio (or someone else) will make an extended battery for the droid like they did for the G1, I have a day and a half of use on my G1 with heavy heavy use and it only cost me 60 bucks to do so. Now I can use bluetooth and GPS whenever, I have my screen brightness up as high as I want, I stream pandora and listen on bluetooth for two hours over the course of a day and spend another hour watching videos with a bluetooth headphone set and I rarely get into the yellow. I haven't seen red in months.