Don't forget to use your vanishing points to construct the whole shape and not just the bottom plane! :) From what it looks like to me, you are extruding your prism from the bottom plane rather than using the vanishing points to construct the top.

Instead of criticism, here's a step by step: 1. Draw your horizon line 2. Place a vertex along the horizon to act as your vanishing point 3. Draw a line perpendicular to the horizon 4. Draw a triangle by connecting the 2 ends of the line to the vanishing point 5. Now that you have a perspective plane, you can extrude whatever you want from it In the case of 1 point perspective, you can simply extrude 2 lines from this plane horizontally in relation to the horizon. This is your front facing plane. To complete the 1 point perspective cube, simply draw another line along the plane perpendicular to the horizon. Now you have a 1 point perspective box :) In the case of 2 point perspective, simply add another vanishing point to your horizon and connect that to your perspective plane as well. This will create a diamond shape, but don't think of it like that, instead see it for what it really is, a 3D wedge shape made up of 4 triangular planes. As far as 3 point perspective goes.. I've never tried it so I can't help you out much there. Hope this information helps :)