The biggest thing is to use credit wisely. If you don't use credit at all you will score low even if you are financially strong.
But things like buying a car or some other big ticket item and making all of your payments on-time helps a great deal.
Using credit cards and demonstrating that you can keep up your payments also helps, even if you pay off everything in 30 days to avoid the interest.
What will kill your credit scores is making late payments. These days it is also a bad thing to be carrying too much debt to income ratios. Even though it seems like it is what creditors want, since they will keep raising your limits.
But high limits don't equal high credit scores.