Some people claim that katakana are harder than kanji. They have a point. The trouble with katakana is that since they’re only used for loan words, you end up not seeing them very much in the course of study. And since they don’t have a meaning, there’s no “hook” to remember them by. Picture a case where every English word that was borrowed from a foreign word – like ennui or schadenfreude* – was written in a completely different alphabet. Might be a little hard to recall letters like z and q, no?
There’s an English sentence – “The quick brown fox jumped over the lazy dog” – that uses every letter in the alphabet in a single sentence. Someone much cleverer than myself needs to make something like this for katakana.
*I coulda picked more pleasant examples I guess?
Lan'dorien's Mysterious Journey 
Ever look at this Tofugu post about Katakana?
http://www.tofugu.com/2009/06/25/strange-katakana-words/
At first when I was first learning Katakana I thought they were just loan words from English, and indeed that is all our teacher showed us. Then I found out like English Japanese borrowed words from other countries and made them Katakana (some had Kanji for them in the past though, like 天麩羅 (テンプラ）。And now there are some Katakana words that don’t even sound like any word you know in your native language and are solely Japanese.
Anyways I think you should just learn them the old-school way with flash cards, or use the Heisig book for Hiragana and Katakana? Since you’re using his method for the Kanji anyways.
Enjoy that Tofugu post.
That’s a good post!
Katakana was one of the first things I learned, as, I suppose, most people do; the trouble mainly is retaining them since I don’t see them enough.
I think I’m going to apply some SRS to the problem.