there are a few things i've learned from painting that are used to give a flat surface "depth"
1. perspective - things get smaller as they get farther away
2. color- colors get more muted and cooler(bluer)the farther they are away(think of mountains and how they get lighter and lighter in the distance)due to the atmosphere and dust in the air etc...
3. detail- we can see more detail up close than we can far away, try reading a newspaper at 10 feet...so edges get crisper the closer we are to something...
4. contrast- things that are closer have a more dynamic contrast, this one and #2 are sort of the same thing... close up shadows are darker, highlights are brighter etc..
so you can use this knowledge to 'push' the depth of your photos even further by..
#1 choosing subjects that repeat at different depths and show how they get smaller...say a fence or road that goes off into the distance...
#2 choosing reds and warm colors for the foreground to 'jump' forward, and cooler lighter backgrounds that 'recede'
#3 choosing subjects with texture and detail for the foreground to take more advantage of the fact that detail and texture tell us something is close...
#4 chose subjects in the foreground that have alot of dark darks and light lights and subjects in the background that are more limited in lightness and darkness...
try not to think of these things as gimmicks, they arent. they are just a few of the things our brains just naturally get used to when looking at landscapes and outdoor scenes and anything else. it's these cues that help our brains figure out what it is were looking at, so we don't get disoriented by all the visual stimulus around us all the time.
There, was that pretentious or what?! lol