How not needing to remember which flats a musical key may have

It is easy.


The A minor scale has all white keys:
Hello - Lionel Richie

The D minor scale or key only B flat.
Back to Black - Amy Winehouse
Stepping Stone - Duffy

The G minor key only B flat and E flat
Adagio in G minor - Albinoni, Giazotto
Intro of album Wish you were here - Pink Floyd

The C minor key only B flat, E flat and A flat
Ανναμπέλ - Ξαρχακος

The F minor key B flat, E flat, A flat, D flat
(The Power of Love - Frankie Goes to Hollywood)


See the patern?
Both key names and flats augment by 4.


Further reading:


Musical Acoustics - Donald Hall