2. Put a fade_distance on each of your light sources.  This makes lights behave much more realistically.  Always do this.

3. Your eyes don't always look horizontally.  They are usually looking up, down, and all around.  How about tilting your camera to do the same? 

4. Detail by distance.  Don't put much detail on that warehouse on the other side of the river.  Instead, concentrate your efforts on the boat in the foreground. 

5. On the other hand, don't put ALL the detail into the boat!  Make the surroundings visually interesting. 

6. Learn what these keywords mean, and use them: #include, angle, filter, inverse, #while. 

7. Sometimes, shadows are more important than objects.  Pay attention to them. 

8. Seldom use white light sources.  Instead, add some color to them.  A little, or even a lot. 

9. Fog is easy to do, looks great, and works indoors and out.  Why didn't you include a fog in your last scene? 

10. Many of these tips involve light and shadow instead of models and objects.  See a pattern?