Lightweight Mavericks installation

What should I do to aggressively lower the memory usage of my Mavericks installation ?

The only things I need to run are XCode and Firefox, everything else can go out the window; I'm not opening any app and yet I already feel the system's slowness, which is understandable because it runs with only 2,5GB of RAM but unfortunately I can't upgrade right now, so what should I disable and where ?