Well you could either place the router in a more central location or you could get a wifi extender. I have not used one before so I am not sure how to set one up.
Signal booster is the only way I know of. I have one, it will pretty much reach all around my property but is very very weak outside of it which is a good thing. The booster I have is a Linksys WSB24. Its about as older than WiFi.