Adding dirt maps to your render

Open your raw render in PhotoShop.

Copy and paste the dirt map you want to use above your rendered image.

Distorting and Transforming dirt map

Distort the dirt map image with Edit > Distort so it aligns with the wall edges.

Use a SOFT LIGHT blend set at 70 -80% opacity so you will now see the staining coming through.

Tweaking your dirt maps

Delete the dirt map on areas where it wouldn't normally appear i.e. windows and doors.

Add a Layer Mask to the dirt map and using the Gradient tool, with foreground colour White and background colour Black, pull downwards.This has the effect of feathering out the bottom of the staining.If the staining is not strong enough duplicate the dirt map layer.

Add dirt maps/staining elsewhere by copying back in the original dirt map and scaling to suit.Again, it is good to add the layer mask/gradient so the staining is more pronounced at the top of the wall.
You can copy this layer around and add colour variations such as red to indicate rust staining,or green to indicate plant run off.
It is also a good idea to add a couple of different dirt maps to add variety.
There are some photoshop grunge brushes available for download but I always find this method much quicker.

You can also use the tutorial on adding grass to raise the level of realism in your scene.