How can a convincing smoke effect be created using a particle system?
A good texture for the smoke can be made using the Clouds or Difference Clouds filters available in Photoshop/Gimp. Start with a transparent background, add the clouds and then erase a randomly shaped border around the image (the square edges of the texture are too distinctive if you don't do this). Finally, set the whole image to low opacity, probably no higher than 50%.
Create a smoke particle material in Unity using the Particles/Multiply shader and add it to the particle renderer. In the particle animator, set the five colour animation values with decreasing alpha (ie, set the first to 100%, second to about 75%, etc, down to about 5-10% on the fifth). Add a small amount to the Y component of the force value - this will simulate rising hot air - and also have a bit of Size Grow. A small amount of random velocity (set in the emitter) will also help. Having set the particle system up like this, you should see some quite realistic smoke.
The effect can be further improved by using more than one smoke texture. The renderer's materials array can accept several different materials, each with its own smoke texture. The main issue with particle smoke is that the individual particles are often too easily spotted. Using several textures will help, as will reducing the alpha of the original image and compensating by using greater numbers of particles.