It comes with practice. The more you journal the more you remember.
I like Robert Moss’ techniques, which include the rather nice idea of going back into interrupted dreams and using the trance state to finish them, or change them if you didn’t like the outcome. You can also use this to relive lost dreams, being in trance brings them back more easily.
Edited to add:
I find it really cool as well, that when you change a dream like this, you forget the original outcome - or, I do anyway - so that you get a real sense that reality is exactly as you want it - not even that it shifted though you remember you fixed it but not what it was before. It feels like a big clue of how that could work for physical reality when I get that far - repeat it to dream it different.
End edit
Another issue is that you’re waking up after the dream is over, and you’ve changed which part of the sleep cycle you’re in and the dream is forgotten before you wake up. Setting an alarm to wake you up mid-dream so you can write it down can help with that.