You should have a look to which programs you installed are running in the background after login, and remove the ones you don't need. The programs would be still installed, but won't run until you need them. In a terminal, type:
You'll see something like this:
Just don't disable any program before knowing is safe (search the names in the web, to see what their function is).
More items you can disable, but this time services. In a terminal:
sudo apt-get install rcconf
Again, these are not programs but services (daemons in GNU/Linux) so search the web to see which ones you can disable safely. For instance, I don't have a printer nor a modem so I always disable CUPS and modem-manager.
Another thing, install preload, an utility that loads libraries into memory according to how often you use them:
sudo apt-get install preload
And that's it, the program will run in the background, you don't need to configure it or anything.
Hope it helps!