The particles is a sensible approach, usually if the trail doesnt have to be interactive I'd simulate a small puff of volume that dissapates quite quickly (emitting from a sphere with some noise for temperature, density and velocity emission). Then instance that simulation on particles with the life of the particles driving the frame to pick.

Then just some noise on your particles will get you something ok. Ideally you would want the flame component to be a short fire simulation which you can attach to the exhaust of the missile.

I don't do much volume rendering with mental ray but it looks like your partices are very flat lit. Can you describe how they are being shaded at the moment. I'd expect that if you have an IBL and a volume shader that it would start looking alot better. Volumes look good when you have some noise in them and indirect shading. If your volume is flat shaped and your lighting is flat, it will look ... flat.

Laurie