The effective range of the WLAN can be extended by observing a few basic guidelines. The wireless signal can be affected by mobile phones, cordless phones, microwave ovens, obstacles, etc, etc. By taking these factors into account, for example, in the placing of antenna, then the adverse impact of these factors can be minimised.
The Netgear knowledge base site provides a lot of useful information on configuring wireless routers and access points, including this article outlining general guidelines that should be observed/tired in order to maximise the strength of the wireless signal and thereby extend the WLAN range.
Further to the above there a number of other means of extending range.
For example simple homemade antenna can be constructed for low costs. Sites giving the details on how to construct these homemade antennas include:
Commercial providers of wireless boosters/antennas include: