This is what Apple says to do in Apple AirPort Networks : Extending the Range of an 802.11n Network:
1. Open AirPort Utility and select the device that will connect to the internet.
2. Choose Base Station > Manual Setup, enter password
3. Click AirPort button, click Wireless
4. Choose "Create a wireless network" from the Wireless Mode pop-up menu, then "Allow this network to be extended" checkbox
5. Select the device that will extend the network and choose Base Saton > Manual Setup, enter password
6. Choose "Extend a wireless network" from the Wireless Mode pop-up menu, choose the network to extend from the Network Name pop-up menu
7 enter network name and password
8. Click Update to update the device
There is also the approach of WDS, Wrieless Distribution System, but each deviice that is added cuts the speed of the entire network in half.