Quick and dirty way of adding depth of field to an image is by creating a gradient blur effect using a blur mask, levels and the gradient tool…

1] Duplicate the image layer you want to add the blur to

2] Layer > New Adjustment Layer > Levels… to create a levels adjustment layer

3] Press ‘G’ to select the gradient tool. Choose the Black to White gradient from the gradient select panel

4] Click with the Gradient Tool in the area you want to remain in focus and drag to where you want it to be out of focus. Use a shallow gradient for a more dramatic blur effect

5] Control click and hold on the layer mask thumbnail to load the gradient as a selection

6] Click the Background layer and select Filter > Blur > Lens Blur (not Gaussian)

7] Drag the slider to adjust the amount of blur and click OK

8]  Ctrl+D to deselect the area.

Finished effect should look like this…

…from this original image…

If you want more realistic depth of field effect or perhaps to recreate extremely shallow focal depth on an area of your image (like with a macro lense) , then you should use the lens blur tool again and choose the reflect gradient type. Will post this, along with the reason why you should use lens blur rather than Gaussian in part 2 of this hint.