Ooh, can I share my mnemonics?! (It’s so rare anyone would be interested, so I have to get it off my chest 
).
ム
Helpful to remember that hiragana for mu is む, which is absolutely a winking cow face saying moo. ム is just a simplified version.
マ vs ス
I was able to remember that they were either ma or su, but not which was which, so: Both are an image of the prow of a ship at sea, but in マ the sea is in the foreground, so that one is ma for mar → sea in Spanish (or marina or maritime or marmaid 
in English). Whereas ス has the ship in the foreground, so it’s the s-sound one. Admittedly not the strongest mnemonic ever, but it does work for me.
More importantly, I have a hot tip for learning katakana. Install Katakana Madness. I LOVE this script because it lets me learn which readings are on and which are kun in a very easy intuitive way, AND since I installed it my katakana reading skills have grown by leaps and bounds. Higly recommended. 