Try to draw them a few times the night before/the study time that you have in the exam. (And if you dont know what diagrams will be in the exam, try to look over all of your topics and see what things would usually have a diagram, i.e if you have spent some time on cells in Sience, then you will likely be asked to draw a diagram of a microscope in the exam)
it's a pain to do sometimes but drawing them out repeatedly can help. you could also draw the hardest ones out really big and stick them above whereever you do your homework, you'd be surprised how much you learn just from looking at them for a few seconds every time you go to sit down
Maybe look over the explanation of your feature and try and visualize a picture in your head of that feature requiring a diagram. Then study the diagram your teacher drew or maybe study the diagrams in the textbook to remember the key things to label in the diagram and to help memorize what it looks like. Also practicing drawing the diagrams could help make it second nature to you.
use acronyms for example if your learning (fat cells , dermis , epidermis) you would remember it with something like "five dancing elephants" obvioulsy most are longer . it doesnt have to make sense infact i remebr it better if it doesnt make much sense