While there are several software and hardware hacks for boosting your Wi-Fi signal (see below), there are also a few simple adjustments you can make to an unmodded access point to get the best coverage. The NY Times says:
Place the base station centrally on an upper floor, or atop furniture, because radio waves spread best laterally and down
Reception will be better if the signal does not have to travel at steep angles and if it doesn't have to go through thick walls, mirrors, fish tanks or anything metal
Place wireless network components far from other devices in the house that can cause interference, like cordless phones, microwave ovens, baby monitors or halogen lamps
Change the default signal channel (usually set to 6) to avoid interference with neighbors' network devices
Place the base station centrally on an upper floor, or atop furniture, because radio waves spread best laterally and down
Reception will be better if the signal does not have to travel at steep angles and if it doesn't have to go through thick walls, mirrors, fish tanks or anything metal
Place wireless network components far from other devices in the house that can cause interference, like cordless phones, microwave ovens, baby monitors or halogen lamps
Change the default signal channel (usually set to 6) to avoid interference with neighbors' network devices