How To Keep Track of Your Collection
Knowing what you have in your collection by keeping an inventory serves several sensible purposes: You will have a record in case anything is lost or stolen, and you will know what you don't have yet.
Collectors kept inventories long before there were computers, making their own lists or using simple inventory booklets available from stamp suppliers. "Paper" inventories still work very well, and can be as simple or complicated as you want. The usual inventory is arranged by catalogue number of the stamp, and may also list the condition of the stamp, when you bought it, how much you paid for it, and any other information you want to include. For covers, you could list them by the country where they were mailed from, and then some description of the item. It's really up to you.
Some of the inventory booklets you can buy, ready-made, are small enough to fit in a pocket, and are convenient to take to stamp shows so you can quickly see if you need such-and-such a stamp for your collection. It's a good idea to take a want list to a show with you - in the excitement of the chase, it's not always easy to remember specifically what you have and what you still need!
Computer-users who also were stamp collectors quickly realized how this high-tech tool could be used in their hobby. Some created their own stamp-inventory databases, while others simply used existing word-processing programs to list their holdings. Specialized philatelic inventory software came on the market in the 1980's, and has become increasingly sophisticated. These programs are widely advertised, both in the print media and on the stamp-hobby sites on the Internet.
As a visual record, some collectors are using scanners to save images of at least their "best stuff," in case of loss or damage, for insurance purposes. If you don't have a scanner, you can make a photocopy of your high-value purchases, videotape them, or photograph them. (While you're thinking about this sort of thing, check your personal insurance to see if philatelic collections are covered, and to what extent. You may want to think about specific stamp collectors' insurance, available from several companies.)