I've just started working on my Grade 6 pieces and I'm making fairly slow progress. I've realised the problem and it's something that has bugged me for a very long time: I can understand a key signature but will never then apply it to the piece I'm playing. I have to almost stop and think of which key to play.

Has anybody got any advice that could help me with this problem please? Thanks for your time.

Start by playing the scale.

Play lots of things in different keys.

maybe you could start writting in the accidentals you are forgetting?

One potential problem with that is that there might be far too many to write in making the music difficult to read.
Also if you do that too much it's not going to help with sight reading where you could easily get a key with several sharps or flats.

You could also try playing well known tunes without music in the offending key, might help to get the feel of it.

Sbhoa is right: if you write accidentals in, it's the easiest way to make remembering them harder in the long run.

Like Inuksuk says, pick out a fairly simple melody like a nursery rhyme and then work out how to play it in lots of different keys by ear. The more sharps or flats there are the better. You will need to think what the actual notes are called - the enharmonics aren't good enough! Once you've done one melody work on another another day. As Sbhoa says, play the scale first and say the correct letters out loud as you play.

Another way is to read the notes of an unfamiliar piece out loud without actually playing it. Think carefully about the key signature before you actually start reading the piece. Have a look at Bach's 48 if you have them maybe.

After a few weeks of doing this everyday, you'll look at a key signature and remember the #s/bs as you play it.

What strategies has your teacher given you?

Tell us how you get on.

You could play the primary chords of that key work out a few cadences.
I find that can help to get your fingers knowing what sort of shapes they need.

Try photocopying the piece and highlighting all the notes which are to be played as sharps - colour always helps

Thank you very much for all of your input. My teacher has so far offered little advice, except from thinking through the scale before playing. I shall work on some of your ideas. Thanks once again.