Hope these help:
www.utahyouthsoccer.net/tt_why8.pdf
www.epysa.org/pdf/USSFBestPractices.pdf

make sure you are doing a lot of fitness. they won't tire out as quick

Sneak your fitness into fun and active games that get everyone involves and so engaged that they don't realize they're conditioning. Don't get caught killing enthusiasm by making them run laps.

Here's a few more links:
www.mayouthsoccer.org/download/443_us_soccer_best_practices_2005.pdf (Great reading through about page 16 on the subject)
www.coachingsoccer101.com/soccercoaching.htm
www.coachingsoccer101.com/avoidlines.htm
www.coachingsoccer101.com/progressions.htm
www.coachingsoccer101.com/whysmallsidedgames.htm

I would also recommend spacing your players out more - and utilize verbal communication & passing. Let your players work. In many leagues defense is used to "hanging out" and defending - and offense runs their tails off - don't be afraid to let the defense work a little and give the offense a break - this is mostly accomplised through spacing.

Hope this helps!