And the ideas are always faster than the hand drawing them, so you probably thought of eco-domes, floating cities and so on, great lighting...and all of a sudden you
find yourself rendering an hour on a shitty looking objekt which you thought was pretty easy to do and when you think what little you just did, you start thinking about the whole picture and
well...it gets really frustrating. But: Such a scene is really really hard to do nicely, because a lot of factors come into play. As an advice i can only add to alnoth that you should
work small, make thumbnails (lots) and tiny tiny studies...valuestudies and black and white studies to evaluate the form, perspective ideas, use sketchup to get a sense of the objects...solve the basic problems.
And most important: Keep it simple. A thought process for your painting could be (before you even touch a paper or stylus):
- what's the desired outcome of this painting, whats my goal?
- what's the level of finish i want to achieve?
- how much time do i have or do i want to spend on this? (For a finished piece like yours i would at least think about 10 to 20 hours or even a week, depending on your skill level and level of finish)
- what are the most basic forms in my illustration (in yours, its two cones standing on top of each other)
- do i have the technical skills to render these forms in perspective?
- what lighting situation will there be? Can i control it? If not, can i simulate it or get reference for it? Can i learn these things i need for this in the alloted time?
- what kind of research do i have to do for the things i don't know how to render?
- the plan "i want to paint something awesome and cool" is mostly applied but might lead to frustration

Spend time there and don't get caught up into details...they actually don't matter that much, they are the icing on the cake, the last thing to make your painting shine. So keep it going, keep practicing...it will get better
@Ambiguity: I should have updated this a bit more often...i read yours when i finished mine^^