For a diffused effect add layers of petroleum jelly to a spare UV or skylight filter. You could go for a slight diffuse or a heavy diffuse, depending on the number of layers that you apply. I mentioned a spare filter, because jelly could be very difficult to remove. Remember to leave the center clear for focusing. I keep one of these ready at all times in my bag.

A few other ways to achieve a diffused look:

1. Breath on your lens and shoot quickly.

2. Hold a very thin opaque cloth or tissue over the lens, but focus first. This will create a heavier diffusion.

3. Have someone blow smoke between the lens and your subject.