This technique works best on scenic photos, such as of trees, leaves or gardens, but feel free to try it on any images you want.

Here is the image I will be working with:

Now create a NEW "Channel Mixer" adjustment layer, to do this, click on the adjustment layers icon at the bottom of your layers pallete, and select "Channel Mixer".

A channel mixer dialog will pop up, just click OK to close it for now.

Now set the blending mode of this layer to "Lighten"

Now change the settings to like below:

Now, your image may not look like mine at this setting depending on the lightings in your image. So adjust your "Blue" and "Constant" sliders around until you get some good results.

But like i said before, this will work better on some images than other, so if it doesnt turn out good on 1 image, try another :)

Here are mine at different Blue and Constant Settings.

Not much snow at all

A little more snow.

PLENTY OF SNOW!