Disable unnecessary services at startup

For example, do you really need Samba for sharing files over the network, or the Bluetooth service to connect to Bluetooth devices on a computer that doesn't have a Bluetooth adapter?

Go to System → Preferences → Startup Applications (Debian and its derivatives). Here you can remove check marks next to the services you wish to disable at startup. But be careful when turning off services. Some applications might stop functioning because you decided to disable a service on which they depend.