I used statistics. Masc hard are most common, followed by hard feminine and i-declension. (neutrals are so like masc that no extra effort needed) And I made up ridiculous mnemonics. Example омstrойmюntal - instrumental. This puts the principle m, f, i-dec singular endings on my tongue ом, ой, ю for instrumental. Also one for dative, same concept. Dative => dуtеvи => у е и put russian vowel endings into english word. I made up a few others, but the nom acc and gen forms were no probs. Will post if anyone wants. From these the soft nouns are usually a spelling rule eg o => e. Most of nine out of ten basic noun declensions are easy to get this way, so that most of the singular forms come from simple one word mnemonics. The plurals are a different animal, and for those I put the whole column into a simple mnemonic. After simple noun ending memorization, I went to the combination adjective + noun endings and set them in mnemonics a different way. The points about animate and inanimate are easy to fit in. The soft nouns are usually easy to get by spelling rules. Then I fit in things like фотография. So I learned them by layers.