Use WPA security. If you don't do this, assume you will get hacked. WEP is essentially worthless. Replace wireless equipment that doesn't support WPA. Seriously. (See Wi-Fi Security)

Use a strong WPA passphrase. A good way to do that is with diceware words. (See What Makes for a Strong Password or Passphrase?) Write your passphrase on a label and stick it on the bottom of your wireless router so you won't forget it. (If someone gets to your wireless router, you are compromised regardless.)
Make your wireless SSID unique. This helps avoid network collisions. A good way to do this is to use your address, phone number, and/or name for your SSID (making it easy for you to be contacted if something is wrong with your wireless network).

Don't bother with SSID hiding or MAC address filtering. They don't do any real good (improve security) but they can cause you grief. (See Wi-Fi Security Myths)
Turn off Universal Plug and Play (UPnP) in your wireless router. Because most consumer-grade wireless routers lack UPnP authentication they are vulnerable to attack. (See Problems with UPnP, Lack of Authentication)

Set a strong password on the administration interface of your wireless router. Again, diceware is a good way to do that.
Turn off remote administration. If your wireless router supports remote administration, turn it off (unless you really know what you're doing).

On unsecured Wi-Fi use VPN (Virtual Private Networking). Otherwise your wireless traffic can be snooped and compromised. (See Secure Internet access in a public hotspot)