Hmm... Detail is a thing that's seldom appreciated. Even I have yet to grasp the ultimate world of detailing just yet, though I like to think I'm getting closer.
An obvious tip is to do a lot of studying in whatever it is you're drawing - reference images are handy, but use them to understand the dimensions and proportions of the object. Do not trace, freehand: that's the technique where you draw an object based on just your memory or imagination.
One thing I will say confidently is that blocking-in has come to help my quite a bit with character and BG detail.
What I'm about to show you here is a drawing I haven't completed yet, in the hopes that my perception of detail may assist in shaping yours.
Blocking-in, from my understanding of it, is when you put in basic geometric shapes in place of large or detailed things you want to draw. My plan is to create a destroyed city BG, so I put some rectangles together in irregular shapes.
From then on my imagination fills in what should be there. I see a set of stairs, my next plan is a door; I see the faces of the buildings, I know I need windows; ect ect...
Though this is still far from completion, using the shapes of the rectangles helped me in perceiving the corners and measurements of the things I want drawn. A lot of the rubble was just experiment, as I did not need to follow the sketch outline too closely, but I probably could not have put it in without something to work with beforehand.
(I'm not one for teaching art, but I hope this helps nonetheless. ^~^)