How do I increase wifi signal strength?

Read: Wi-Fi base stations: Setting up and configuring an extended wireless network (802.11n)

You will mislead yourself by attempting to "strengthen" a Wi-Fi signal. All base stations already use the maximum power permitted by regulatory agencies.

The above KB article explains how to add one or more AirPort base stations to extend the range of another AirPort base station. Extending the effective area of your wireless network is the goal... not increasing the wireless signal you already have, which will prove fruitless.

Read it carefully. The best solution is called a "roaming network":

Wi-Fi base stations: Setting up and configuring a roaming network (802.11 a/b/g/n)

You can use an Airport Express (< $100.00) as an extender by placing (plugging into power) the device within the range of the wireless router, between the router and the desired connection location. It's just a matter of setting up the Airport Express as a range extender.