There are several ways
- Multiple Access points, each fed by hard wired Ethernet (Such as Ethernet over Power), this is actually the easiest
- Wireless Repeaters, this normally requires compatible hardware, the Netgear WG 602 will do this and I imagine a quick Google will find others.
Other ways can be improving the aerial of your router, going more directional, hacking your wireless hardware to increase the power.
Jen