Debt to income ratio is a concern whenever you are applying for a loan or anything where you credit is being considered. That same ratio takes into consideration your available credit since you can be $0 in debt one day but $50,000 in debt the next day. Thus, making you higher risk if you have way too much available credit.
So, what you want to do is the following:
1. Distinguish between your "good" debt and "bad" debt. Good debt is anything which increases in equity or holds value such as a mortgage or student loans. Bad debt included credit cards, car loans, and anything else which decreases in value over time or cannot potentially increase in value.
2. Figure out what your debt to income ratio is by considering your bad debt plus your available credit divided by your income.
3. Once you figure out 1 and 2, you will want to shoot for getting your debt to income ratio to be around 30% or lower for the best results.
4. Once you complete step 3, start placing some money on each of your credit cards but keep it minimal. Even $50 will do just fine. Make sure it stays on the card for at least one billing cycle. That way, every month your credit report will show that you are using your credit as opposed to a zero balance card which is equivalent to no credit.
Lastly, remember that no credit is just as risky as bad credit in the eyes of many lenders (this was told to me by the Bank of America branch manager). So, you do not want to rack up tons of years with no debt to your name. Having a few bucks on each of your CC's is what accumulates the valuable history that lenders like to see when it comes to your credit report.