Drench a paper towel in vinegar and lay the iron on it for a good five minutes, then start rubbing again, more and more crud will come off. Now place the baking soda on the drenched paper towel, place the iron on top of it and start ironing.
Use Salt To Clean Your Iron Pour salt on your ironing board (Perhaps cover with a pillow case or something to make clean up easy) then iron the salt with steam option off. The dirt on your iron will stick to the salt, making for an easy clean
Cast Iron seasoning, cooking, and cleaning guide : castiron