A lot of students report that learning choreography is like trying to hold water in their hands. It goes well a few days, but then completely slips away on the next day. This is typically because they were only able to store the movement as a series of snapshots, rather than a series of transitions. The brain is better at remembering the transitions than the poses. Try and forget about where your arm is and where your foot is, and instead remember weight transfer, rotation, sinking, etc., and the water will stay in your hands a little longer.
One strategy for improving memory is sub-vocalizing (whispering to yourself) as you go through the dance. As you step back, turn left, lower yourself, you can tell yourself that you are stepping back, turning left, lowering yourself, and the two mechanisms reinforce each other. I also find it useful to divide a dance up into phrases, much like the phrases in a paragraph of writing. If you practice each phrase until you have it, and only then string phrases together, you are less likely to be overwhelmed and less likely to have the whole thing fall apart if you forget one little thing.
A big mistake is repeating the entire choreography each time you mess up. This drills just the beginning part while keeping the rest of it loose. Try to start at the exact spot where you start to get lost. For example, if you have trouble with the approach into a turn, do just the approach and the ending several times until it is instinctive. Try going backwards. Repeat the last few counts first, then add the counts leading into it. This way you can create a series of security from the end, leading backwards into the beginning.
Even a targeted 15 minutes can be productive if you use it wisely. Walk through the choreography slowly, without music, and note changes in weight and direction. Practice 2 or 3 short phrases to music and repeat them until you can get through. Try to run the piece once or twice and allow for small mistakes. You’re aiming for retention, not perfection. End with a relaxing walk-through to cement the information before you stop.
If you have a moment where your brain just goes completely blank, do not stop and/or have a nervous breakdown. Continue on in the timing of the choreography, and try to find your way to the next place that you can remember in the piece. Sometimes if you can just keep moving, your body will remember the choreography again. The body learns dance phrases through momentum, so if you stop moving completely, your body won’t remember the phrase either. Through practice, your ability to find your way again and continue the choreography is just as important as memorization, and your ability to recover in the middle of a phrase will get better and better.