If you use de mineralised water or rainwater to water your terrarium (which you should be!), you shouldn't need to clean the inside of your terrarium as unlike tap water, no water stains form as de mineralised water and rainwater lacks the salt that causes these marks. For the outside, use a lint free cloth and fresh water to clean your terrarium. Remember, it is a living ecosystem and harsh chemicals may harm it - although a bit of window cleaner on the outside glass is fine.