Kana were derived from kanji. They are simplified versions of some of the radicals found in kanji (radicals are identifiable recurring parts in kanji such as 力 in the kanji for cooperation 協 or 女 in the kanji for like/love 好.) Any good book on kanji will have a foreword or a whole chapter on kana origins, recognition, and writing. I personally don't know of any way to make kana easier to remember except writing them over and over and over and over again just as they do in Japanese schools until you can write them right and remember them. Same goes for kanji. There are a few mnemonic tricks to remember some of the more physical kanji, but once you reach the abstract, I feel the mnemonic trick to be a hinderance. The book Kanji & Kana: A Handbook of the Japanese Writing System (Tuttle Language Library) is a good bet if you can't remember kana yet. They have an extensive foreword on the subject.