On my old Linksys router I tried optional high-gain antennas (larger rabbit ears that replaced the originals). They did make a difference. Also, make sure your router is up as high as you can place it and not behind any massive metal (such as a fridge or tucked between a PC tower and a metal desk).
I've read several on-line plans for directional antennas, mostly a shaped framework holding aluminum foil (I recall one was a modified Pringles can). Never tried those but they made sense, assuming you had the router in one corner or end of the home and only needed a boost in one direction.