I have little mental techniques for memorising key signatures. They basically involve seeing them as little pictures which make them more unique and indistinguishable from each other. For instance, the A major looks like an upside down A. The F# major looks light two jets flying downward, so in my head it's F#ghter Planes. The C# one looks like waves on the sea so See Ships/C-Sharp (sound association). Others are more like constellations where the letter is imprinted on the signature. E and B loosely place over the shape of them. The closest i can describe it is like it's some form of Synesthesia.
I've also used an IOS app that helps to remember them. I'll do a review of that soon.
Also writing them out by hand helps. Maybe get some blank manuscript paper and fill it with you drawing them out. And say what the notes are in your head as you do it. Once when I was younger I was forced to write the lord's prayer by hand one hundered times. I'm not religious but if I am in church, I can recite that thing word for word and not even know why. That's what you want to do with music. Imprint it in your brain so that it's just there.
I wont show you my flat image associations. Would be fun to maybe see some of yours! (Okay I'll give you one. Bb has two flats so looks like bb).