Go to Editor -> Add Build Phase -> Add Run Script Build Phase
Go to Build Phases in the project navigator and edit Run Sript.
Change Shell to /bin/bash and paste the following script:
#!/bin/bash
buildNumber=$(/usr/libexec/PlistBuddy -c "Print CFBundleVersion" "$INFOPLIST_FILE")
buildNumber=$(($buildNumber + 1))
/usr/libexec/PlistBuddy -c "Set :CFBundleVersion $buildNumber" "$INFOPLIST_FILE"
Don't forget to change the Build number found under General in the Identity section from 1.0 to 1
Have fun! :)
I found this tutorial on Cocoa Factory