Particle System Follows Player and Can Change While In Game?

So I've attached a particle system to the player in my game. When the player moves I would like the particles to emit in the opposite direction that the player is moving. For example, if the player was going up, the particles would be emitting to the south. This would give it an appearance of the player being propelled by the particles. Any idea on how this would be done? I know how to add a particle system and play around with it. Though I am not sure as to how I would create this in C# or Javascript. Thanks in advance.

 

Could try with Emit() and setting the velocity direction on that:
http://docs.unity3d.com/ScriptReference/ParticleSystem.Emit.html

Or, just rotate the particle system on keypress and then doing Emit(amount) only.