1. enable WPA or atleast WEP security if possible...
2. use mac address filtering
3. use other than default channel
4. turn off wireless if not in use
5. place your wifi access point somewhere in the middle of the house.

The important of security is only about someone could get into your pc and do anything with it. beside with wireless, anyone within the wifi range can use it.

the above mention is only a starting point..