You can buy comercial cleaners especially for irons - check out your local hardware shop, or you can get them on line. The best ones are a paste that you squeeze onto the hot plate and then spread with a cloth.
mine is in a tube like a lipstick, you rub it on and wipe it off, you can get impregnated cloths as well. If it is stuff melted on it an you can't get a cleaner, then try cranking it up to full heat, full steam and ironing over a wrung out but wet piece of old cotton towel, make sure that it is 100% cotton
i use a bit of fine wire wool, brings off any residue very easily but wait for the iron to go completely cold before you do it so that the residue has completely hardened.