I know some will say "READ THE RULES" but I will state it here for those that have a difficult time finding such things.
Each week there are about 10 Cr cards up for grabs in the ELO tournament. To be eligible you must place in the top 100 players for the week. This level can vary but realisitcally you should aim for over 1400. The Cr cards are given out either at random among those players or using a formula I do not understand. There are also a number of rare cards given out at random to the remaining players. To see which cards are available each week, click on the list to display the current standings on your home page. It will be on the right and a little way down the page. You can also find out where you are in the standings.
OH yeah btw its not "get a high amount of ELO points and participate" its get a high amount of elo points BY participating...the tourney is a week long and is judge acording to your points (which go up if you win and down if you lose)
there is also much more then that. For full rules look here: http://www.urban-rivals.com/game/rules.php?show=elo