If you have a removable antenna, you could replace the omni-direction stick antenna with a direction Yagi antenna. The problem with a yagi is that they work on a 70° tranmission arc hence it will transmit only in the direction that you point it. Not the best idea but it may be an option.
Don't know the wireless device you have but some are shipped with the transmit power set to about 30% of maximum (eg Linksys). There may be a setting in the interface that allows the transmit power to be increased.