I tend to be a little on the picky side about my cast iron skillets that are used to prepare some mighty fine venison, and want to keep the aged seasoning on them. Here's a few tricks that may help yours last forever, which my Teflon-Inclined wife had to learn also.
1. When cooking, make sure to heat no higher than medium setting, which allows the cast iron to slowly absorb the heat and cook without burning the food. Turning the heat to high, especially without any oil in the pan, can actually take off some of the seasoning on the bottom.
2. Always use either wooden or plastic utensils when cooking. Metal utensils tend to scrape the seasoning off of cast iron that doesn't have heavy seasoning from years and years of use.
3. Avoid cooking acidic foods, such as tomatoes, in the skillet until it has a good seasoned layer on it.
4. Never run cool water over a hot skillet. You can warp or even crack a hot skillet with cold water. If you run water over a hot skillet for ease in cleaning, be sure the water is hot.
4. Do not scour a skillet that has a thin seasoning layer. You will just take off what is already there. Also, stay away from dishwashing liquid. The soaps strip off the seasoning. NEVER put cast iron in a dishwasher. In a matter of minutes, you will take off what has taken years to put on the
skillet.
5. Don't allow your skillet to sit in a wet sink for days at a time. It will rust.
6. When cleaning your skillet, always be sure to dry thoroughly. I run my skillets full of hot water and allow them to sit in the sink for 15 minutes. This helps break up any food, and allows the heat to transfer from the water to the cast iron. I clean the skillet with a dishrag and wipe thoroughly dry with a towel. Since the heat from the water transferred to the skillet, any dampness on the surface evaporates after wiping with the towel. The heat also allows me to wipe the skillet with a paper towel and Crisco, which melts and provides a thin oil coat which adds to the seasoning and prevents rust.
If you ever want to recondition skillets that are Old and Rusty, it is not that hard. Take it to someone that has a sandblaster, and that old Rust and seasoning will come right off. Sandblasting allows you to clean off everything that is stuck in the pores of the iron. Wash the bare cast iron in warm soapy water(dishwashing water). Rinse off the soapy water and dry the skillet thoroughly, and wipe down the whole thing with a thin layer of Crisco. Preheat the oven to 350 and bake the skillet upside-down for one hour. Make sure you have a pan on the rack under the skillet to catch any grease that may fall off. You don't want the oven to catch on fire. After the hour is over, turn the oven off, leave the oven door shut, and allow the skillet to gradually cool inside the oven. Once it is completely cooled, you will have an initial layer of seasoning, and are ready to begin cooking with the skillet. It will take many moons and many pounds of venison before your skillets begin to look anywhere near as good as Grandma's.