Is it directional? Go for something where the bog-standard aerials can be unscrewed and a directional one plugged in instead. I know my Draytek can do this, cost-cutting on most home ones these days means they're usually stuck on.
Then pick up something like this: http://thewifishop.net/wi-fi-aerials/high-gain-directional-wifi-aerial/prod_10.html
There's only so much aerials can do though within the legal transmit limits. If you have something that can run alternative firmware like DD-WRT you can boost this up to the legal maximum (and beyond if you really wanted). Can you put in any repeaters / range extenders somewhere in the middle?