The Ask Mr Credit Card blog provides some tips to raise your credit score that are counterintuitive:
By having several credit cards, you are able to show you have a long credit history with many borrowers.
Having several cards can also help your credit utilization ratio. The post also mentions having a car loan can help if it doesn't have too much effect on your credit utilization:
Not having a car payment hurts the variety of credit that I have, yet not owing that money helps my credit utilization.