If you use de mineralised water 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 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.
Plants may require pruning if they begin to outgrow their container or become unshapely. Remove the plant's growth tips to encourage it to increase in density. Remove any dead or diseased parts of a plant from the terrarium.