A surprising number of "broken" cartridges that arrive at our Brighton workshop are not broken at all - they are dirty. Thirty-year-old contacts, attic dust and the occasional spill of something nobody admits to are usually all that stands between a cart and a working save file. Cleaning and storing them properly takes about ten minutes per game and adds decades to their life.
Cleaning the Contacts the Right Way
Forget anything you have read about a pencil eraser. It works in a pinch but leaves rubber residue and abrades the gold plating. Use isopropyl alcohol at 99% purity, applied with a lint-free cotton swab. Wipe each contact twice, let it air-dry for a full minute, then test. For Game Boy and SNES carts you can usually clean without opening the shell; for an NES cart with truly green contacts, a Gamebit security screwdriver lets you open it and clean both sides properly.
What Not to Use, Ever
Water leaves mineral deposits. Window cleaner contains soap residues that re-attract dust. Compressed air is fine for dust but useless on oxidation. And please leave the toothbrush out of it - bristles drag grit across the contacts and cause the very damage you are trying to fix.
The Save Battery Question
Carts with internal saves - Pokemon Red and Blue, Zelda: Link's Awakening, the early Final Fantasy releases - contain a small CR2032 battery that lives roughly 15 to 20 years. If yours predates that, the save is on borrowed time. Replacement is straightforward but requires desoldering, so unless you are confident with an iron, take it to a workshop. The cost is small. The lost save file is not.
Storage That Actually Works
Three rules. One: keep cartridges off external walls - condensation collects there. Two: avoid direct sunlight, which fades labels permanently within a single summer. Three: store cart-side up, not flat, so any dust falls past the contacts rather than into them. Acid-free polypropylene sleeves keep loose carts looking clean and prevent label scuffing during storage rotation.
Looking after old cartridges is a quiet practice. You do it once, properly, and a cart that has worked since 1992 will keep working through 2042. The technology is honestly more durable than people give it credit for - it just asks for ten minutes of attention every couple of years.