a range extender is the way to go but if it's not supported and the $$ ain't in the budget, you can try a few things:
1) reorient your antennae (this is most effective if you have more than 1 on the box). set 1 in a vertical and the other at an angle. this will tip the antenna coverage slightly and it may put the client into a better location in the antenna lobe.
2) relocate the router to a more central location in the house.
3) getter a higher gain antennae.
Veritas Principium Libertas