There is a trick to seasoning cast iron, especially for the first time. First off, scrub it really well with hot water and dish detergent to get off any coating that the manufacturer may have applied (since you've already used yours, you might be able to skip this step). Next, coat the cooking surfaces generously with lard (yes....better for you than crisco) or peanut oil, or another fat with a high flashpoint. Then you need to heat it until the fat smokes (seriously). Since it's a waffle iron, I'd set it in the oven at 400, rather than trying this on the stovetop (set the iron in a pan that will hold it comfortably, to keep the hot oil from dripping into your heating element and causing a fire!). Take the pan out and let it cool until you can wipe the excess fat out with a handful of paper towels, and that's it. Don't use soap to wash it! Very hot water, only, and heat it to dry. You may have to repeat the process a time or two, but it works like a charm, even on old, ill-kept cast iron.
(I just ordered a waffle iron, which is what led me here....definitely stealing some waffle recipes!!)