Short of moving the router or getting a different one with more range, you can get a wireless repeater.
I installed one at my in-laws house. It works well.....you just have to understand how it works....it will actually show up as a different wireless access point....so instead of connection to 'my home', you would connect to 'my repeater'.....
BTW....how far are we talking?
If you have an old router...instead of buying a repeater...get a new one with more range....and while your at it...get one with NAS. Basically, NAS capability lets you plug a USB HD into the router and it is now accessable by any computer on your network. Sort of a cheap and easy way to have a network drive.