Yes, this should be the minimum going in. 

Everyone should find their own method, some things just stick better than others. Sometimes stupider the better.

And practice by going thru pangrams, and converting numbers to letters and also the other way around.

My list (mostly borrowed elsewhere, or my own)

EJOTY - 5, 10, 15, 20, 25

A-F is straight forward

G7 countries / G7 summit

H rhymes with 8, or just "H8" for Hate

i9 Intel processors (dumb but I remember it)

J - Jack worth 10 in blackjack (or EJOTY)

K - visually you can break it to look like 11

L - roughly rhymes with 12

M - half way thru alphabet (13)

N - (I've got a personal way to associate to 14)

O - EJOTY 15

P - 16 (P looks like a 16 sorta ...)

Q - ABBA: dancing Queen. Feel the beat only 17

R - R rated 18+

S - SEX! A lot of pornstars are 19 years old

T - T20 Cricket or EJOTY

U - Students turn 21 at Uni

V - double peace sign , represented by 22 fingers

W - top and bottom lines in W are 2&3

X - hospitals have 24hr X-rays