I started looking for simple explanations on the subject and here's what I found:
1. "The drawing of ellipses is controlled by rectangular perspective" (source). As I understand it the easiest way is to simply draw a rectangular or a square and then squeeze the ellipse/circle inside it (see here).
2. Also there is a more geometrical approach. Using a horizon line and many many center lines creating loads of little triangles and mid points one can create perfect circle-ellipse. But the summary of this method is almost the same: Draw a square/cube and create your circle/ellipse/tube using it.
I didn't use a ruler in any attempt; maybe that's the reason why everything is so uneven, even weird looking at times.
After browsing and checking some more sites I came to the conclusion: when in doubt - draw a rectangle and fit your circle/ellipse in it.
But I did find more interesting sites with some great explanation.
1. Mike Sibley talks about the importance of drawing an accurate ellipse: "The eye of the viewer can detect any irregularity in a shape that it fully understands". Thus, it must be perfect.
2. Automotive Illustrations says that to change the center of the ellipse one should simply move its center line.
3. Craftsy does a very nice job showing first how to NOT draw an ellipse.