LIGHTING.
Variance in lighting makes for a dynamic photo, and can add to the required 3D effect. A dark foreground, or a dark object in the foreground, with light colours in the background help to create that effect, especially in landscape photos.
RANGE OF COLOURS.
This can add further to the effect of lighting, and helps break up the photo into planes, as some lighting effects will do.
Visualise a cloudy but sunny day, with patches of light scudding over a landscape of varying colours. I have found scenics of the Yorkshire Dales often have this effect.
FRAMING.
This is not referring to the actual photographic frame as such, but to the act of looking for objects, such as overhanging trees, to create a natural frame. This can also help toward the 3D effect.
BLACK AND WHITE.
Include a touch of black and/or white if you can, in order to bring out all the other colours. Have you noticed how a bed of deep red flowers is enhanced by clumps of white amongst the red. Just a tiny bit of white especially draws the eye and breaks up colour masses. Sheep and Holstein cows, small in the photo, can do the trick, or a white building in the distance, at one of the third points, is advantageous.
DEPTH OF FIELD.
This can define how sharp your photo appears, presuming you are not of course prone to camera shake. A tripod can take care of that problem .
What it means is the distance in front of and rear of the subject you have focussed upon that will appear sharp. Anything outside that area becomes increasingly fuzzy.
Depth of field can be increased or reduced by the f stop you are using.
An f 11 gives a greater depth of field than a lower f stop such as f 8 or f 5.6.
It can also be affected by the distance from the camera to the subject focussed upon.
10 ft. from camera to subject gives less depth of field than 30 ft. from camera to subject for instance.