I'm going to go through how I learned to differentiate between the Japanese Hiragana characters "れ" ("re") and "わ" ("wa"). The only difference is that "re" has an outward hook at the end of the rightmost stroke, whereas "wa" continues inward.
Practice
Duh. Just keep working. But, the nature of a "leech" is that practice just doesn't seem to work. You try over and over again to pound those terms into your head, but they bounce right off, like... bouncy balls off pavement. We need something soft and sticky, like gum, not springy rubber toys!
Change Study Habits
This is the obvious next step. We have to change things up! Change is good for your brain. And your wallet.
I'd do practice sets with just the "r" sounds: "ra", "ri", "ru", "re", "ro". I was easily able to identify the "re" sound, then, but when I'd go back to practicing every character, I'd mess it up again. I'd mess up less (so it helped, a bit), but I'd still mess up. Bleh.
Okay, what's next?
Change Study Habits
Oh.
If you're working with Anki flashcards that go term -> meaning
only, you're only getting half the benefit. Now, now, don't go just meaning -> term
only, either! Do both simultaneously (use the "Basic (and reversed card)" in Anki)!
Aw, man! I'm still making mistakes? What now?
Change Study Habits
Seriously?
This is the one that cinched it for me: practice just "re" and "wa"... and go fast! Go as fast as you possibly can! Don't worry about mistakes-- just go fast. Of course, this initially just led me to look for the direction of the hook at the bottom of the character. That's not bad, but when studying other characters along with these two, it could cause some problems. Solution? Add more characters. Slowly. But still go fast.
The thing is: don't stick to just one method. Keep changing it up!
Obviously, the study method that worked for me, I guarantee is not going to work the same for you. Experiment, change your study methods often.
EDIT: If all else fails, write an answer on a StackExchange site detailing your problem and your attempts to solve it. It can help more than you know... ;)