The best way to keep it clean is to wash it with warm water and mild soap (like dish soap), then rinse it with clean water as soon as you are done using it. The Camelbak brushes are designed to fit the nooks and crannies of their reservoirs, but you can use a regular sponge if you need to. Either fill it up with clean water and toss it in the fridge or prop it open to dry. The Camelbak drying tool works really well and is worth the money. If you really don't want to invest in one, you can also stuff it loosely with newspaper until it's dry. If you don't, it WILL mildew. Use cleaning tablets or 1 tbsp bleach diluted with 1 liter of water about once a month if you use your reservoir regularly, or at least at the end of the season if you only use it once in a while.