I agree with paul. You just need to repeat, repeat, repeat. Write them down many times, say them out loud, and you can also make a flashcard with something like: 3rd decl. m/f (on one side); all the case endings (on the other). The more ways you try to come at the problem, the more ways your brain will have to think about it, and the greater it will stick. Mnemonic devises are always easy ways to remember things (if you can come up with a good one), but the 3rd decl. has three different ways it can be declined depending on m/f, n., or i-stem, and I think that would just be too much.