Then select "Services"...
...and then choose "Webserver" and check that the "Allow control of XBMC via HTTP" option is selected (i'e' it has a red dot beside it).
Then go back to the home menu and select "System" and then "System Info"
This will display the "Summary information" screen. Make a note of the IP address (in my case 192.168.1.107)
That's all that is required in the way of setup on your Raspberry Pi!
Now go to the App Store on your smart phone and search for and install the XBMC app. There is an app available for both iPhone and Android. The following screenshots were taken from my iPhone, but the setup process should be similar for Android users. When you launch the app on your smart phone you'll see the following screen.Select "Add Host"...
...and then enter the details for your Raspberry Pi. All you need to enter is Description, IP address, Username (pi) and Password (raspberry) and then select "Save".
On the following screen select the "XBMC Raspberry Pi" entry....
...and then choose "Remote Control.
That's it! You can now power down your Raspberry Pi and unplug the USB hub along with the keyboard and mouse and when you power your Raspberry Pi up again you can control it from your smart phone.