Getting more heat from radiators?

putting metal behind the radiator will only absorb and retain the heat, it wont necessarily push it out into the room. I use a small flat metal basket (it looks like a big flask) filled w/water that is designed to hang over the back of the unit. it heats the water and evaporates it into the air which not only increases the humidity but also helps distribute the heat. You can buy them at a home depot or Lowe's.

You can put a reflective piece of material on the wall behind the radiator to reflect some heat. Radiators have to have air circulated behind it and then through it to radiate heat. As long as you don't block the air flow you should be all right

what type of radiator do you have, cast iron, baseboard, or sectional in the wall? the two most important things are; they be clear of dust and have free airflow over the coils for the latter two types; and that you bleed any air out of them, which ever style you have. i have found that an old snowbrush works good for cleaning out dust from any type i've had in the past, then i take the shop vac to them. we have also painted the back wall of sectional types with a silver paint like rustoleum, but use a can of it and brush instead of the spray can. my brother used spray cans once and the overspray got all over everything. good luck, hope this helps.