How can I make sure my WI-FI signal is not being "stolen"?

Try the following 3 steps:

1. Add authentication to your wireless network.  use WEP with 26 bit
key - this will make it difficult for someone to use your wifi
network.

2. Do no broadcast the SSID.  Normally you just find the list of
wireless networks and click connect.  If you configure your router to
not broadcast the SSID, then you will not see your network in the
list, but you can still connect to it by typing in the name and asking
your computer to remember it so you dont have to type it again and
again.

3. The wifi router usually has configuration to alter the power. 
Instead of blasting out at 100% of the power, reduce the power of
antenna.  This will reduce the range of antenna.  Adjust it to some
value thats just good enough to reach your entire house.  Minimize
reach, so less people are physically in distance of using your wifi
signal.

Besides this, check router logs - of which clients have actually
connected.  This will tell you if someone is stealing the signal or
not.

The short answer is "you can't" unless you start MAC address filtering
and applying 802.1X authentication systems. WEP is insecure and not
the best way of fixing wireless.

usrhlp

well, a good intruder doesn't leave any evidence , but basically you
can always access your router configuration, if you cant it means the
admin password changed , so somebody used your connection and you need
to master reset your router and reconfigure it. thats an attack from a
newbie. Each router generates a very informative log file and you as
an admin could configure that option for keeping the log information
as long as you like. Its simple , if you know about the legal users
connecting to your router , you can read all their activity in the log
file, any unknown MAC address or USER ID listed there in an intruder.
So if you even noticed that the log file is deleted , its an action
from an intruder , change the password and secure your connection
more.
These are some methods for basic to moderate users.

At higher levels companies use gateway monitoring apps, which are
famous as WI-FI watch dogs, somehow you can even use this apps in home
if you have 2 computers and a static IP.

There is also another method geeks using it, we call it honeypot
trick, its a long process , if you are interested you can read about
it here:
http://www.netstumbler.com/2004/02/16/wireless_honeypot_trickery/

and don't forget when you get rid of intruder its the right time you
concern about your wi-fi connection, read some papers , fortify your
gateway, and prevent any further attacks.good luck

Like the other posters have said. You can't stop a wireless signal
from being broadcast. The only "sure" way is to build a faraday cage
[http://en.wikipedia.org/wiki/Faraday_cage] around your home to stop
signal from leaking out. However that is pretty much unpractical. Like
the other poster said, reduce the signal to limit the range.

As your signal is broadcast, given sufficient time, any cracker will
eventually be able to get enough information to crack the encryption.
Various methods of encryption are simply deterrents but some are
better than others. MAC filter, WEP and WPA encryption can be bypassed
and cracked.

Another solution given sufficient resource and expertise is you can
setup secure channel inside your Wireless LAN using VPN, PPTP or some
other authentication. This is what most big enterprises and
universities do. This won't stop people from getting on your wireless
but it will prevent them from using your Internet.

Hope that helps

Hi,

Do the following:
1) Disable your SSID for guest mode Infrastructure mode.
2) Go for 128bit encryption key
3) Change your SSID password atleast once in 1 month.

Keep doing this and hopefully you won't face any problem.

Such a simple question doesn't deserve so many convoluted and
incomplete answers.  Here's some straightforward advice:
http://www.onguardonline.gov/wireless.html

[Meta-advice: search for answers via google before asking questions]