How do you stop programs from running in background?

[mookiemoo]

Finally switched over from windows mobile TP2, and have the epic. So far I love it, BUT, everytime I hit advanced task killer, there is a long list of programs running, lowering my memory. How do I prevent these programs from running in the first place? When I shut them down via ATK, later on, I see there is another long list of programs up and running. In many cases, there are programs running that I did not even select to use.