There's a lot of factors that affect RAM usage. Graphics drivers for one (without Nvidia drivers installed, Xorg uses less RAM, but with it installed, it uses double the Xorg RAM usage on my pc), graphics hardware for another (Intel graphics come built in with the kernel, so naturally my laptop with Intel GMA graphics uses much lesser RAM than my pc which has a Nvidia GT430 even though they both have 3GB of RAM), RAM plays a role (the lesser RAM you have, the lower RAM it uses--less caching going on), and services that start up automatically on boot. ALSO, the version of Debian you're using also affects the RAM usage--on Wheezy, my idle RAM was ~110MB--so in general, RAM usage usually goes up with further Debian releases.
I disabled all useless services using rcconf
(you run this program in a terminal); there's also a GUI program to disable services called bum
(short for boot-up-manager).
List of services I have disabled
Apache2 (I can manually start/stop/restart this)
Bluetooth daemon (I don't have a bluetooth enabled phone so I don't need it)
Exim4 ( https://help.ubuntu.com/community/Exim4
Fancontrol (my motherboard can do this just fine automatically, no thank you)
Hddtemp (my case is cool inside as are the components, there's a rear fan)
Keyboard-setup (something to do with setting up what country's keyboard you want. I use English, I have no need for this)
OpenVPN (don't need it)