A simple way to memorize a 24 word seed.

Write your words like this:

Cover up all words with a paper and slide it to the right, revealing 4 words at a time. Only work with groups of 8 words at the time, as indicated by the vertical lines. For each group, try saying the next 4 words in your head before revealing them. If you can't, reveal them anyway by moving the paper, say them, and then cover up all words in the group of 8 again. You will likely have to move the paper back and fourth a lot during the first practice round.

Do 3 trials a day – not more, cramming is a waste of time. One trial means that you have worked with each group of 8 until you have recalled the words just once when covered up by the paper. The first trial should take less than 3 minutes if done right! Speed will grow proportionally from that. You want to be able to remember the words at least as fast as you are able to read them.

I'm still doing some testing and will provide more data later. The preliminary results suggests that total practice time should be less than 15 minutes if spread out over a week.

Note: I'm not suggesting that anyone only store their seed in memory. I view this as one backup among several, and whether it's a good idea to memorize a seed depends on personal circumstances.