Lifehacker Gina Trapani wrote a nice article accumulating the top 10 Wi-Fi tips that can increase your signal range and make your wireless network secure. Tips include things like better router placement, extending your signal by using a repeater or a bridge, using  Wireshark to sniff your wireless network packets passing through, enabling WPA Encryption to make your network much more secure than using WEP, enabling MAC address filtering to restrict your wireless network even more, stop SSID (Network Name) broadcasting, using NetStumbler to detect other wireless networks with information about how secure they are and also for war-driving purposes, using portable routers share a wired connection in hotel rooms for instance, a nice youtube video showing how to use tinfoil to increase your signal range, and of course using DD-WRT Firmware to enhance your routers usability and adding more features making your $50 router into a $600 one. It also includes instructions on how to flash the firmware to your router.

Web Worker Daily has a nice article about securing your wifi network. It includes basic tips and detailed instructions on advanced techniques.