My top tip is to only ever play in silence, with nobody and no distractions around, sitting or standing upright. These three things have added 200 points to my rating.
I also find that noticing from intuition (a strange move by your opponent might be a trigger, or a tight position where you want to play strategically might be another) when to spend more time on a move sequence helps a lot.
Taking a mental note that you spent a bunch of time on one move and need to play more quickly is another improvement. But this is also an art that comes with experience.
I was just thinking about this. I've had spells (as recently as last night!) when I do want to play, but my mind is pretty tired or my mood is off and I end up dropping 50 rating points in a session because I'm sort of half-assing it. I should probably just play unrated games at these times (or just puzzles).
I also find that noticing from intuition (a strange move by your opponent might be a trigger, or a tight position where you want to play strategically might be another) when to spend more time on a move sequence helps a lot.
Taking a mental note that you spent a bunch of time on one move and need to play more quickly is another improvement. But this is also an art that comes with experience.