Hard to say without knowing more about you and your routine. I think the comedown has alot to do with it.
Try not opening your eyes until about 10 - 20 minutes after you have come down. Just sit and think about what happened and try to re-experience it from start to finish. Often times you will encounter the same thing over and over again in a trip and this will help you remember it easier.
If you would like more concrete practice, try remembering your dreams first. From what I have experienced and read, remembering a dream is very much like remembering a DMT trip; it's very fleeting and faded. Make a dream journal, try to visualize places you've been and things you've seen with your eyes closed before sleep.
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