This may sound silly, but since I invented names for the weird symbols under the buttons, you know, the smiley face and angular lines you see, I've never had to write a passcode down, unless it was longer than 6 or 7 entries.
Somehow, having a name for each makes it easier to concentrate on what the code is, and hold it in your mind. It's also fun to try and remember whilst playing a section of the level. Don't ask me why ^_^
I will go through the signs, as seen in bank 15 of the level editor, starting with object 13.
13 - "S" - I call this S because it's like a graffiti-fied letter S.
14 - "Golf" - This made me think of a ball on a tee (the right side), with the curved line sweeping into it representing the swing of the club
15 - "Rasher" or "Bacon" - Press button... Recieve bacon... Nuff said.
16 - "Battery" It's a square box, with a broken circuit, and a plus sign. I also see the rightmost tip as a minus sign. Thus, this is without question a battery.
17 - "Zigzag" Simply because nothing more suitable sprang to mind.
18 - "Right Atari" It looks like the right side of an atari sign (albeit a flipped one)
19 - "Smiley" Obviously...
20 - "Square Smiley" or "Corner Smiley", it's clearly a face, but with the mouth pointing to the top left corner, and it has a unibrow, or something.. (!?)
21 - "Left Atari" Same reasoning as object 18, only the other side of the logo.
Since using these I haven't had to write any codes down, though they might only work for me, because they were my first impressions. Try using your own if these fail you.