When you come to move, and a thing you're planning to move is still in a box since the last move, throw it out.

If you are keeping a thing 'because it might come in handy' and the occasion arises when it WOULD have been handy except you forget you have it, throw it out.

On smaller purchases I note that I have a danger zone of between £3 and £8 where it's easy to just spend money without discernible benefit (it's no coincidence that this equals about a coffee and a bun in Starbucks). So I have a rule that unless it's something I actually need Right Now, I make a maximum of one such purchase per non-working day.