Whenever I teach the subjunctive formation, I first teach the students the regular formation, then when they have mastered that, I teach them the 20 verbs that form irregularly in the subjunctive. They sort into three categories: boot (two) stem, irregular stem and just plain irregular.
To have them help remember which verb goes where, one of their assignments this unit is to come up with a mnemonic device to remember. An anagram, puzzle, song, anything.
I must share this gem that I just received from a student:
The Verbs of Bel-Air
In the west of France
Born and raised
Listing two-stems is how I spent most of my days.
Venir, payer, mourir and voir,
Appeler, jeter, prendre and boire.
Then a couple of verbs who were screaming them
Started using irregular stems
Faire, pouvoir, savoir, vouloir and aller
And then I decided to go to Bel-Air
I rolled up to the house about 7 or 8
And I yelled to the weird verbs,
"Yo etre and avoir!"
Looked at aie and sois since I was finally there
And then I listed the verbs that lived in Bel-Air
My students are so creative :)