Have you ever wondered why objects that are far away from you appear lighter and bluer and objects that are closer to you appear darker and warmer? This is the effect of aerial perspective and is caused by moisture and particles in the atmosphere which affect the light you are seeing. For instance a range of hills in the far distance will be a pale bluish tone whereas the ground in front of you is warm green or brown. Logically of course the distant hills are also warm green or brown it’s just that they appear different from our view point. By imitating aerial perspective you can achieve an impression of depth in your paintings which will make your work appear in 3D instead of being flat. This sense of recession will lead the viewer into your scene.
Blues can be warm or cool, for example cerulean is a cool blue so if you want your hills to seem far away you must choose a cool blue, cobalt or ultramarine are warm blues. The same with reds, alizarin is cooler and will appear further away than cadmium red.
Edges of course are also blurred and less distinct in the distance this means your shapes will recede. Make sure your far edges are soft and your closest edges are the sharpest.
Have a close look at your painting are the cool blues with soft eges on your distant shapes and your warm dark colours with defined edges in the front. If not, it's relatively easy to add some dark darks in the foreground or lift out some colour in the distance (important - wait until the paint is dry then try gently lifting out with a natural sponge.