Yes, keeping your phone in excellent condition goes a long way towards maintaining high resale and trade-in value. You should keep your iPhone in a case at all times - and always add a real glass (not plastic screen protector). As iPhones and Samsung Galaxy phones get more expensive, the resale and trade-in value has been going up as well. For $25 now, you could add several hundred dollars to the value of your phone when it's time to upgrade.