Try grouping the nouns according to their definite article, and create an image collage representing each of the nouns (say from 8 -13 images) per collage. All items in a single collage would be either der, die or das (not mixed). Get the images representing each noun from a Google Search. You will be surprised at the retention rate as you visually remember each of the nouns in a collage. And, the nouns don’t have to have any relationship other than their definite article. e.g. Create a collage with the following on it: a girl, in bed, reading a book, with a reading light and she is resting on a pillow. What have you learned? das Mädchen, das Bett, das Buch, das Licht and das Kissen and das Schlafzimmer… and if she has her cellphone, das Handy, and a dress, das Kleid. So that collage becomes a mental image that is easy to remember and you now know 8 definite articles for the price of one image collage. I’m over 550 nouns with a retention of 99%.