Dream until your dreams come true.
Then work like hell for every chance that comes. I remember when Guillermo Gonzalez was going to be the great Mexican-American soccer hope coming out of Southern California. He played on U.S. youth teams - was the midfielder feeding the ball to Freddy Adu. He got drafted with a nice fat salary to the Los Angeles Galaxy while still a teenager. He didn't have the work ethic of Jorge Flores, though. He gained weight and was out of the league even before his contract was up, having only played in a few games.
For all the people who point out how some players in MLS get pitiful salaries - don't forget to take note of those who sign contracts that are sizable, but never pan out. There are bargains and there are busts.