Most ruined cast iron is not ruined at all. It is just mistreated in a few predictable ways. Avoid these and your pan will last for generations.
1. Letting it soak
Water is the one thing cast iron cannot tolerate sitting in. A pan left in the sink overnight will flash-rust by morning. Wash it promptly and dry it immediately.
2. Air-drying
Even a quick rinse leaves moisture in the pores of the iron. Always dry your skillet on a warm burner for a minute, then wipe a tiny bit of oil over it.
3. Cranking the heat from cold
Cast iron heats unevenly and slowly. Blast it on high from a cold start and you get hot spots and warping. Preheat gradually over medium for a few minutes.
4. Cooking acidic foods too early
Tomato sauce, wine, and citrus can strip a fragile seasoning. Once your pan is well established this is fine, but go easy in the first few weeks.
5. Using too little fat
A dry pan with a young seasoning will grab onto eggs and fish. Be generous with oil until the surface is bulletproof.
6. Storing it with the lid on
A sealed lid traps humidity. Store the pan uncovered, or slip a paper towel between pan and lid.
7. Believing the soap myth
A little dish soap will not destroy modern seasoning, which is polymerized oil, not a layer of grease. Scrub it, dry it, oil it, and move on.