Recently on an IBNS forum, I responded to a post from Asia asking "what is the optimum storage of banknotes?"
There certainly are a number of things that a currency collector should consider, such as quality currency holders, and I have paraphrased some of the responses into a few categories and included my own comments for the readers benefit:
1. Temperature & Humidity - It should be on the cool side (less than 70 F) and most importantly - consistent. You do not want to store your collection in an East-facing room with windows that heat up with morning sun or a room with drafty windows that gets cooler in Winter. Humidity can also be a problem as paper naturally absorbs the ambient moisture. The effects of inconsistent humidity will cause paper to become more brittle over time. You should consider keeping desiccant gel packets with your collection wherever you store it.
2. Ultraviolet (UV) light from the sun will fade paper relatively quickly and can also make it more brittle. Do not leave your banknotes out in the sun for any length of time. Ideally storage should be in a room with no windows. You can study your banknotes with Ultraviolet Lamps without any concern as you generally only do this for a few minutes at a time.
3. Clean hands are essential. The introduction of natural hand oils and other contaminants can cause staining and damage over time to the banknotes. Keep your food in the kitchen and wash your hands before working with your currency. Ideally, use cotton gloves to handle the banknotes themselves.
4. Acid-free storage materials. Of course my personal favorite. Currency holders, currency album pages, and cases MUST be free of plasticized PVC and chemical stearates. The material used in many inexpensive 3-ring binder pages, emits hydrochloric acid over time and will damage the notes. Be careful to only use SAFE Currency Holders or Currency Albums that are made with acid-free materials.