Inevitably, when you go through a lot of stamps over time, you will come across damaged copies which are not really suitable for your collection. Once you have decided to discard a stamp (in the case of more difficult-to-find issues) or whenever you find damaged copies of more common issues (which is easy!), you can put them aside for future use. Whenever you are ready to undertake the job, you can use these in the following way: I keep my good "spares" or extra copies, of which there are often tens if not hundreds available, in a shoebox (actually a box sold for the purpose of storing photos). Each stamp has its own envelope (this is a large job today with most countries, but it is a very effective way of keeping track of spares - I get the envelopes cheaply at the "dollar shop") and on each envelope I put a copy of the stamp in question.

You can develop this concept much further if you like. I number my envelopes using Scott, so the stamps are not so difficult to find, but with the vast array of stamps available today, it is still sometimes a bit tricky to locate the stamps at once (without looking them up in the catalogue every time, I mean). So I also put extra copies of the more common stamps on the internal dividers which I use to split up every 100 Scott numbers within the box. I am even thinking of putting stamps on the outside of the box in future to make it still easier to locate the right box by sight, but I am concerned that any stamps thus employed will end up getting knocked about so much that they will eventually become detached. Perhaps if I cover the entire box and then lacquer it somehow, it might work. But I am not there yet.

Anyway, the old "stamps on envelopes in the shoebox trick" is one useful way of using damaged stamps and organising a large selection for the purposes of exchanging and so on. In addition, with more difficult varieties (British Castle issues come immediately to mind), you can keep details of what makes each variety special (and identifiable) on the front of the envelope.