When handling banknotes, it is recommended to use a pair of good quality stainless steel tweezers/tongs, preferably one without pointed or sharp edges. Avoid using your bare hands as you risk staining the note with oil, dirt and grime, or in some cases, fingerprint marks. Once a banknote is stained, its value will decrease tremendously.
To keep banknotes in the best possible condition, you should use Mylar/vinyl holders which is acid-free and contain no harmful softeners. Common household products such as plastic wrap, plastic bags, window envelopes etc. should not be used for storing banknotes. Most plastics contain PVC, which deteriorates with time and heat, releasing acids and gases which will migrate into the paper. This will result in the notes appearing to have been soaked in oil, or so brittle that they shatter at a touch. Use only products (such as Mylar and other inert plastics) which have been tested and approved for long term banknote storage. Improper storage will result in the note being toned over time.
Banknotes should be stored or displayed away from direct sunlight or powerful spotlights to avoid fading. Repairs should only be made with products that are stable. “Magic” tape and other plastic tapes will leave permanent stains on notes.
There are individual Mylar holders which look like plastic envelopes and are usually sold in packs of 50 or 100. You can also keep banknotes in regular paper envelopes but it is not recommended as they are more liable to get damaged while being handled.
If you really want to get fancy, there are special albums with associated Mylar holders. Each page can hold 1, 2, 3 or even 4 banknotes. Some of the more reputable brands in the market include Lindner®, Lighthouse® and Prinz®. Most professional numismatic collectors and dealers use individual Mylar holders housed inside vinyl stock sheets for maximum protection, handling and storage.