A Microsoft Paint drawing i made using the Two Point Persepctive to achieve a three dimensional image. Obviously MS Paint uses large pixels so the drawing isn't as smooth as it would be on RMD. I used Paint just because the screen is larger for this demonstration.
First draw a long horizontal line as near to the top as you can. Then place TWO POINTS along the line. Unless you want a very narrow perspective place the Points far apart from each other as i have done.
Next, from each Point, draw a long line about halfway down the page and connect them to form a Large V.
From the end of the V draw a vertical line which should fit about five times from top to bottom of the screen. Then connect the end of this line to the two Points as shown here.
Now this is the hard part. If you wish to make a rectangular shape like i have done, you have to draw a vertical line up from the lower two lines at different places. If you wish to make a cube then you must draw your vertical lines from the same distance from your first vertical line. Whichever you do you must remember that the left vertical line and the right vertical line must connect to the right and left respectively.
Now you must remove the guide lines so that you are left with a perfectly shaped cubic-rectangle. All that is left now is to decorate the box to create something more exciting.
(Disclaimer:I did not create this drawing myself, i found it via google. The rights belong to kingfisher.ednet) Here's an example of what you can create using the 2 Point Perspective. Obviously on paint the colours have turned pixilated when converting to a jpeg (technical stuff) but using this method on RMD should be quick and easy for getting the right perspective in drawings.