Finding the next Key Date Coin is what all the coin collecting addicts try to do.  The primary reason for collecting key date coins is they are the hardest to find, most rare and often times are much more valuable than your standard common date coins.  One of the biggest factor is finding the next key date coin is going to be it production numbers.  The less of a coin produced the less that will be avaiable for any collector to get his hands on it.  If you want to speculate as to what will be the next key date coin you should definately become familar with the production number put out by the US Mint or the mintage numbers from the Country's coin of your choice

 

Here are some example mint production number for the 2008 State Quarters

State Quarter Production ** 
State Denver   Philadelphia   Total ***
2008 *  1,920,600,000
Oklahoma  194,600,000  222,000,000  416,600,000   
New Mexico  244,400,000  244,200,000  488,600,000   
Arizona  265,000,000  244,600,000  509,600,000   
Alaska  254,000,000  251,800,000  505,800,000   
Hawaii       

 

As you can see the Oklahoma quarter has the lowest production numbers and will likely be more expensive to get your hands on in the future.  Buying these lower produced coins when they are new to the market could possibly net you a nice return down the road.  The production numbers for the Hawaii state quarter havn't been released yet, however the coins just hit the market.