Found a useful video about the German mixed prepositions
Hallo Leute, https://www.youtube.com/watch?v=XQFWH_1PUs8 Its short and gets right to the point, it helped me understand dative and accusative in general. As for cases, in general, I found this site: http://www.bbc.co.uk/schools/gcsebitesize/german/grammar/prepositionsrev1.shtml helpful. Even made my own study aid for Dativ, since there wasn't one : BAS MAN VZG
BEI, AUS, SEIT, MIT, AUßER, NACH, VON, ZU, GEGENÜBER Like the name of a firm that makes musical instruments. Another thing that can help is to download a sticky app for windows and write your memory aids and questions for the cases. These stickies being the first thing I read when I start up my laptop really helped me memorize the prepositions.