According to this test, I forgot like 20% of all the hiragana and katakana that i knew 
gotta re-learn them.
@hideki
Put them on a wall.
These are for my son when having a bath. He's half Japanese.
And there another (bigger) hiragana set on a while wall in his room.
@hideki
Another additional option: pick up some sticky notes and write names of things with a marker on it, in hiragana or katakana. And put those notes on the things.
Drawers, fridge, chairs, ... All the things you find out.