Step 1: Black & White
This tutorial works well with Black & White images so if you have a coloured image open it up in Photoshop and go to Image>Adjustment>Black&White to convert your image.
Step 2: Add Gray
Create a new Layer (Layer>New Layer) and rename it 'grain'. Go to Edit>Fill and when the new window opens pick the 50% Gray option. Click OK and your new layer is now filled with Gray.
Step 3: Make some noise
This Gray layer is what we are going to create our grain from so with the 'grain' layer selected go to Filter>Noise>Add Noise. Make sure the Distribution option is set to 'Gaussian' as this is a much more realistic grain pattern. Also ensure the 'Monochrome' option is checked. Adjust the slider until you're happy with the amount of grain present. This, of course, will change from picture to picture but around 15-25% should be a good starting point.
Step 4: Blur the grain
Unlike true film grain our image is a little harsh so a little blur is needed to soften it. Go to Filter>Blur>Gaussian Blur. Set the radius so it softens the grain but doesn't remove it completely. Once you're happy click OK then go into the Layers Palette and change the layer mode from Normal to 'Linear Light'. If we were just stopping with one layer of noise 'Overlay' would work fine but we are going to be adding a second layer of noise. For this reason, make sure the opacity of this layer is reduced to around 80%.
Step 5: Repeat
Repeat step three but reduce the amount of noise you create this time. Back in the layers palette, try changing the blend mode to 'Lighten' and reduce the opacity of the layer to around 60%.
If you want to enhance the darker areas of the image further, brush over your chosen areas with the Burn Tool with an exposure of around 30%.