One of the most prevalent mistakes to avoid in a LOR is not providing your recommender adequate time. You must give your recommender at least a month to prepare an attractive LOR for you. Notify them well in advance and provide periodic reminders so that they can work on your LOR. We're afraid you're in trouble if you approach them at the last minute and hurry them up hoping to be given an outstanding LOR. The haste would undoubtedly reflect on your LOR, reducing your prospects of admission to the university of your choosing.