Wireless 802.11b and 802.11g WAPs (wireless access points) support a
range of up to 150 feet indoors and 300 feet outdoors.The 802.11a protocol is one third of 802.11b/g.Signal interfeirence from microwaves or cordless phones can also affect the range.
The newer 802.11N gives you 14x the speed of 802.11G & 6X the range.

Move the router off the floor and away from walls and metal.You should also place your router in the center of your home or office.If you router is at one end of your building then the other side will get weaker signal.You can also change the channel of your wireless signal.This can cause problems if you have other neighbors that use wireless.If you and your neighbor use the same channel then you can loose signal and power.

You should also update your wireless devices firmware.This includes your wireless card and your router.Just go to your manufacturers site and look for the updates and follow the instructions.You can also buy a wireless repeater.