The I-Beams found in construction sites are obscenely heavy, and work like a charm. The way to do this is to grab one of them, and then find a section between two buildings with a good amount of space.
Have Superman's back to one of the buildings, but 5 or 6 steps in front of it. Every time you press to throw the beam, he pulls it back first. If you are close enough to the building, the beam will hit the building and you'll just drop the beam. Now you just have to rapidly tap and he'll keep dropping and picking up the Beam at a good rate. You'll also have to take 3 or 4 steps forward after a good 10-12 drops, because each time he's moved a little closer to the wall. Keep this up for a half hour or so, and you should get it in no time.