Ok, maybe that's the key (less than 30%)
To maximize your score at a point in time, have less than 10% of your utilization report. It doesn't matter whether you spend $10 or $1,000 per month, as long as you pay it down before the statement date, the credit bureaus won't know what you've spent. The credit card company only reports your balance every month and utilization percent is a MAJOR factor in determining your FICO score. So, as Bacon said, just get a rewards card and use it like a debit card then pay it off before the balance posts on your statement every month.
Also, if you're looking to boost your score, look into a "credit builder loan" through a local credit union. You don't get the money upfront but rather put a certain amount of money in an account every month and you get to keep the money at the end of the term.