(1) Set up MAC address filtering on your router, i.e. only let your machine connect to it, not everyone on the street with wifi enable machine.
(2) Disable SSID broadcast on router when you have all connection running.
(3) Use strongest encryption available on your router and machines, i.e. WEP 128 etc, if your equipment support WAP, it would be better. This will prevent snooping.
(4) Turnon firewall if your router has this function.
(5) Change wep key regularly.