You can achieve this using Soft Body or Cloth physics with gravity disabled or turned right down. I prefer Soft Body since it's more controllable and more suited to this.
Start by creating your bag mesh. It doesn't need to be complicated - just needs to capture the general geometry of the bag :
Now enable Soft Body physics on the bad. You'll need to make some changes to the default settings. Start by disabling the Soft Body Goal since we want it to be free floating. In Soft Body Edges increase Pull since we don't want the back to stretch too much. You might also want to increase Friction in the main Soft Body properties to damp down the motion (as if it's moving in water rather than fluttering in air). To make the bag puff out a bit you can enable Soft Body Self Collision and increase the Ball Size nearer to '1' (this will make the vertices push apart) but also decrease the Stiffness so that they only slightly push apart. Finally, in the Soft Body Field Weights turn the Gravity right down so that the bag 'floats'.
Add some fields - such as Turbulance and Vortex to generate some interesting movement and you should have something like this :
Blend file included