I assume you are using your Raspberry Pi as a HTPC by running Raspbmc on it. (Here is how to do that.) I also assume the Raspberry Pi is connected to your home wireless router or home network.
Raspbmc can easily be controlled from a Android smartphone that is connected to your home wireless network.
Configure your Raspbmc as a webserver. To do this, follow steps 1-4 in this post.
Power on your Android smartphone, turn on its wifi and connect it to the home wireless network.
Open the Play Store (or Marketplace). Search and install the XBMC Remote application.
Open the XBMC Remote application and add the Raspbmc as a XBMC Host by providing its IP address, HTTP port, username and password that you noted in step 1.
XBMC Remote should be able to talk to your Raspbmc. If not, check your wireless router settings.
In the XBMC Remote application, choose Use as Remote Control and enjoy controlling your Raspbmc to play videos, music or pictures.
Tried with: XBMC Remote 0.8.8-beta1 and Raspbmc RC3