Let's forget paper and calculators this moment! I really enjoy to resolve all kind of calculus only with my mind. I have prepared these simple but powerful mental exercises and a trick i use to do it faster and faster.
37 x 42 , 78 x 36 , 342 x 18 ,
To turn it easier ,simply do it like this in 37 x 42: (35 x 40) + ( 2 x 40 ) + ( 2 x 37 ) , doing this you will reduce the time you waste solving problems, because you work with lower numbers.
78 x 36 can be: (70 x 30 ) + (8 x 30 ) + ( 6 x 78 ) , have you understood? Resolve the calculus and you will see that the results will come really fast.
Just use this technique on the way you think that is better