They are very solid in my head right now, but I used to forget cases pretty often when I was learning. The main thing for me was to train the cases a lot until they stuck. It's not elegant or anything, but it worked for me.
Basically the way I learned ZBLL was, I would open up
https://tao-yu.github.io/Alg-Trainer/ (scroll down for help), and then choose a set of 12 algs to study. I would start training myself on the algs immediately (I never used any alg sheets) using the simulator cube, and I would continue to train myself until I could get around 20 in a row correctly. The next day, I'd sometimes have forgotten some of that set, so I'd retrain myself on them until I could get 20 in a row again. If I felt they were solid at this point, I would move on to the next set. If not, I would continue to train myself on the set until I felt I was.
So basically, I just drilled them into my head using brute force. The virtual cube in my trainer helped me drill the cases a lot more efficiently because it allowed me to focus on one thing and one thing only - I didn't need to switch my brain to other tasks such as scrambling, solving and looking up algs in Jabari's alg sheet.
Personally, I found most of the T, U and L sets stuck pretty well after training them for one day, and then doing some review the next day. The other sets would often take longer than this since I found them harder.
Anyway I'm not sure if this method of training is for everyone, but it worked very well for me. Probably the best way to find out what works for you is to just learn a couple sets, and see how you fare with them. By the time you've finished one or two sets of 72 algs, you'll probably have found a pace that works for you and that minimizes your rate of forgetting algs.