Have you ever looked at an object at an antique store or auction that you thought was totally, breathtakingly UGLY -- and asked yourself, "How could this possibly be worth as much money as it is?"  Most people have been mystified at one point in their lives or another about what makes an object -- even an ugly one -- collectible.  There are several factors that contribute to the "collectibility" of an object:

1) Historical value.  Historical value can be realized in many ways:
        Age. The age of an object may, but does not always, contribute to historical value.  Think of it this way: Rocks are millions of years old, but how much would you pay for a rock I picked up out of my yard this morning? 
         Provenance.  Provenance refers to a proveable historical connection to a famous or at least known person of the past.  A shaving brush, for example, would be much more valuable if you had substantial evidence that it belonged to a former governor of your state, and even more valuable if you could prove that it belonged to former U.S. president George Washington.  The key here, however, is proveable -- often by a separate document that came with the item, or by a photograph, or documentable chain of ownership.
       Dates, marks of manufacture, etc.  These increase the value of a collectible by proving age and by giving a context to the object.  One example of a collectible china would be the hand painted Japanese china with an "Occupied Japan" mark.  Nippon china from Japan was manufactured from the years 1890-1921, while Occupied Japan-marked china has an even smaller window of manufacture -- 1945-1952.  In addition, the context of Occupied Japan-marked china has historical significance as the Post WWII period. 
       Distinctive styles of manufacture.  Similar to dates and marks of manufacture, distinctive styles provide a context of both age and the prevailing culture of the time.  A style thought of as "retro" today, the "boomerang pattern", originated in the 1940's and reflects the aesthetic of the Atomic Age. 

2) Rarity.  Rarity may occur for a few different reasons:
       Very few of the items were made.  Some of the most noteworthy examples of this rarity result from mistakes in manufacture -- such as coins minted with errors and soon removed from circulation.  A similar rare item, the Beatles' "Butcher" album cover, resulted from a controversy over the potential offensiveness of the item and its subsequent removal from the market.
       Very few items survived in pristine condition -- or at all.  Older children's toys may be collectible because few items survived years of child-induced wear-and-tear.

3) Capriciousness.  There are just some unpredictables in the collectibles market.  For example, I remember my mother selling a brass bed for $50 -- six months before the first Godfather movie was made.  Because a brass bed figured dominantly in that movie, the price of brass beds shot up to $600 soon after.  How could she have predicted that?  She probably could not have.  However, there are some factors that, if you observe them happening, might lead to educated guesses as to what the next collectible fad will be:
       Tragedy/Notoriety.  Tragedy brings out the best -- and the worst -- in human nature, and the collectibles market is one place to spot some of the worst.  Immediately following September 11, 2001, World Trade Center memorabilia became much more collectible, adding a gruesome feel to many collectibles markets (including EBay for a very short while).  Notoriety has a similar effect on the collectibles market, as is evidenced by the brass bed -- Godfather connection mentioned above.
      Celebrity endorsement.  Jadeite, a type of Depression glass, became much more popular when Martha Stewart extolled its classic appeal.
      Timeliness.  Did you know that this year (2006) marks the 40th anniversary of Star Trek?  It's no accident that a good amount of memorabilia is soon to be auctioned off by New York auction house Christie's Rockefeller Center.  The collection catalog alone is priced at $500 US dollars, and the catalog lists a first season Star Trek: The Next Generation Starfleet dress uniform for an estimated value of  $3000 to $5000.

In addition, the value of a collectible can reflect several of these factors.  An authentic piece of Chippendale furniture reflects the Neoclassical spirit of the late 1700's, the artistry of Thomas Chippendale's designs, and a rare find.  If you could prove it belonged to one of America's Founding Fathers (provenance) and had figured prominently in Benedict Arnold's trial (notoriety), you'd have covered all the collectibility bases and have quite a collectible on your hands!