Learn to sculpt! For me, a few hours making faces from clay was all it took to teach my mind the mapping from 3d to 2d. This forces you to think from 2d to 3d by taking an image you have in your mind and forcing you to build 3d support for it.

Then, do still life sketches of what you sculpt, no matter how bad. This seemed to reinforce the other direction for me.

That and play Descent 2.