Keep your utilization painfully low, but use each of your cards every month.
Wait, and be patient.
- The Zen of "Gardening" 
Meanwhile, apart from maintaining a spotless payment history, possibly the most attractive thing you could do from a lender's perspective is negotiate or job-hop your way to a higher salary. While the scores are important, so are debt-to-income ratio and net worth. That's why most lenders ask questions that target these two factors on their applications.
Don't take that to mean that I am disparaging your current salary. I have nothing but respect for any working member of society, and I make it a point to never judge a person by a number. But lenders make a point of judging people by exactly that: numbers.
As someone who interviews a lot of job applicants, it has been my experience that few people are asking for what they truly deserve. You may be surprised how readily you can get a raise even in the current economy, and how much it can improve your ability to both secure and manage credit. It's one of the best outside-the-box solutions to financial growth I know. And it is kind of bizarre to call it an outside-the-box solution, but as I said, few people actually do it.
I negotiated a raise and a promotion for myself in the past year, and I consider it no accident that afterward Amex finally welcomed me back into their fold after 2 years of denying my applications. Give it a try...you have nothing to lose!