How to send data from a smartphone to Arduino
I have been using a bluetooth module - they can be had for just a few dollars (depending on which country you're in). I've got an HC-06 which works great : http://www.dx.com/s/hc-05

It has 4 pins: VCC (connect to +5V on Arduino), GND, RX and TX (connected to RX and TX, digital pins 0 and 1 on the Arduino).

You will probably not be able to use Bluetooth and USB at the same time - both will be sending to the Arduino at the same time, and the communications will get garbled.

I have an app called "Bluetooth Serial Controller", which lets me send commands over bluetooth.

If using this, think carefully about what will happen if your phone goes out of range of your arduino.