So simple*FOLLOW THE 1,3,5,7,14,28,40 RULE*
First of all, you should not actually memorize a reaction but understand it.
BUT
If you have problem in understanding it I have a solution to memorize them.
Just make a list a reactions you need to memorize.
Revise and rewrite those reaction after 1 day.
Now revise reactions after 3days, then revise again after 5 days, 7 days so on.....
IF YOU ARE HAVING A THEORY EXAM NEXT DAY. replace "days" with "hours" in above lines. AND THIS METHOD REALLY