Debian :: Memory Usage On A Squeeze System?

I use a Debian Squeeze system running off a flash drive, i.e. based on a custom Live image running in persistent mode. It runs great and I am grateful for the existence of Debian . However, I have a question. A lot of the machines I use this pen drive on are quite old, often with 512 MB RAM and old processors. I specifically built my system using XFCE and lightweight apps off an initial live image using the standard-x11 package list (basically just Xorg with drivers and the base system). At first things ran very well, blazing fast even on the oldest systems and could comfortably run Firefox along with LibreOffice side by side (I need LO as all of my colleagues use Word docs, often with track changes, which Abiword can't handle properly). However, over time, I've found that memory usage has risen, tot he point where Firefox is now automatically killed on the older systems every time I start LibreOffice.how does one figure out why memory usage is going up? I've checked for inessential services and turned them off with "insserv -r". I've used only lightweight apps, as mentioned before. Are there other general tips on reducing memory usage?