At your router,if your using wireless, Disable SSID Broadcasting and use WEP-TKIP encryption instead of WPA. Disabling SSID will stop your network form being visible to computers looking for any available network. Just remeber what your SSID (network name) is so that you can still connect to it. WPA can be cracked rather easily. WEP-TKIP is a bit more secure with less overhead then WPA or WEP-AES.
Also be sure to disable remote router management from outside your personal network. This will keep people from changing things from the WAN (internet) side.
PM me if you need any more info 🙂