Several settings can improve the security of your wireless network.  Numbers 1. and 2. are very important.  Numbers 3. and 4. are highly recommended:

1.    Change your SSID.  The factory default is typically something like "Linksys" or "tsunami".

2.    Disable broadcast of the SSID, this prevents other wireless devices from seeing your network.  This is also called "beaconing".

3.    Add Mac Address filtering.  This allows only the client devices you select to get on your network.

4.    Add WEP encryption.  This encryption can be broken but is takes some effort by a proficient hacker.  Changing the WEP key defeats hacking attempts.  It takes about two Gigs. of data exchanged to hack WEP keys   (about 2000 minutes of continuous high speed internet downloading).  Use of WEP also prevents client hijacking. 

5.    Use static IP addresses.  192.168.0.1 is typical default base.  Consider any other numbers in the 192.168.x.x range.

6.    Use directional antennas, and lower the power to meet only your needs.

7.    Review access logs to see if any intruders are logging in.

8.    Use strong passwords on your internal systems (upper and lower case, numbers, special characters).  Disable file sharing if not necessary.

9.    Connect your wireless client through a Virtual Private Network.  These systems add security but also add cost and lower performance!  If you are accessing important information such as work data, this may be necessary.  Contact your system administrator for VPN settings.

10.    A new security standard will replace WEP, watch this page for more information.  There are a range of new standards, but these seem to be all based on changing or rotating WEP keys (see #4 above) or including some form of encryption such as the VPN method (see #9 above).