You need to be good at mathematics. If you are good at drawing and designing that goes a long way with you not only during but also after selection. One more important thing is your perception of and capability to design and recognize 3-d objects. In entrance exam they often ask you a question in which you are given a 3-d object and you have to identify which one among the four given 3-d figures (one of which is the same figure as in the question rotated to a certain degree) is identical to the one given in the question. It determines if you can visualize how a given 3-d object would look when rotated.