Tickets for the match—which start at $25—are available via Ticketmaster, and it's highly suggested that you purchase them immediately. According to the LA Galaxy website:
A very large crowd of over 85,000 is expected for the LA Galaxy vs. FC Barcelona game at the Rose Bowl, this Saturday, August 1st. As a result the Rose Bowl and the LA Galaxy encourage guests to arrive early (prior to 6 p.m.) to take advantage of the opportunity to bring a picnic or tailgate on the Rose Bowl grounds or adjacent golf courses.
Most parking for the Rose Bowl ($20 for cars, $40 for buses, RVs, and limos) is available at two nearby golf courses, but the walk could be "as much as 1.5 miles"—more reason to arrive early. You can also take advantage of a free stadium shuttle, which departs from the Parsons Engineering parking lot (100 W. Walnut Street, Pasadena). The ride is 10 minutes, will drop you off near the Gate B entrance, and runs continuously from 4:00 PM until 90 minutes after the game ends. Note that parking at Parsons is $20, or around $6 at nearby lots.
While attending the game will require some planning, the excited buzz surrounding the matchup suggests that your efforts will be worth it. For more detailed information on parking, including directions to the Rose Bowl, visit the LA Galaxy website. Being prepared will help you avoid headbutting your steering wheel in frustration on the way to the game.