Well, there are patterns to help you memorize. One pattern I found that no book seems to mention is this:
accusative singular: am, um, em, um, em
accusative plural: as, os, es, us, es
The m changes to an s in the plural for masculine and feminine endings. If it wasn't for the u to o change, you could say the vowels are all the same.
Also, I'm working on a word chart. Instead of just charting out the endings, I'm trying to chart out words alphabetically in all the cases. The 5th declension will be a problem because I don't I will find a fifth declension noun for every letter from A through L.