Make it darker by creating eye-catching contrast

First and foremost, try to add more contrast to your painting. Often, inexperienced artists are hesitant in using tones fearing that their paintings will appear too dark. But, artworks tend look too dark due to the dearth of tonal contrast. The lesser the amount of contrast, the more monotonous will a picture appear. The lesser tonal contrast, the greater will an artwork appear with a single shade of gray. The dark areas of painting should make it darker so as to make other areas look lighter and brighter. This contrast provides a higher level of realism to an artwork.

Create a solid foundation before prior to adding detail

In order to create beautiful works of art, the painting needs to be built up with back layers before adding details. No matter what medium you use, you need to layer in the tones and put the detail above the foundational layers. While making an acrylic painting, work from back to the front. While creating landscapes, you need to follow this step. So, you need to paint everything that is in the background first, such as the land and sky far away from the painting's front. All objects in the foreground should overlap those in the background.

Next, build up the colors and apply some of the textures on some objects like tree trunks and foliage. You also need to pay attention to composition to ensure that your painting has a perfect balance. Once you are done, start creating the details. Add a layer upon a layer of texture and color. You can make use of a sponged technique to create an illusion of foliage. Irregular edges of sponge tend to vary as per application of paint. So, it doesn't create repetitive spots which a brush normally does. 

If you are using this technique, you will need to have patience. It takes a lot of time to create paintings with believable details. Since it is an illusion, make it a point to step back and view your painting from a distance of around 5 feet from time to time. Then, you will have the desired effect.