If you've got a fee-free checking account, you better hang onto it for dear life.

In the past two years, the number of free checking accounts have fallen from more than three-quarters to less than one-half. In 2009, 76 percent of checking accounts were free. By 2010 in had fallen to just 65 percent and it declined to 45 percent in 2011, according to statistics in an article from The New York Times.

That decline can be attributed to banks' efforts to impose new fees that will offset losses from changes in government regulations. And while most banks do give customers a way to avoid the fees, typically either by having direct deposit or maintaining an minimum balance, determining whether you're eligible can prove tricky.

Ron Lieber, an economics writer for the Times, looked at the trend and proposed that banks create tools that allow people to easier track whether they'll be subjected to fees — perhaps even adding such a tool to their mobile apps.

But Lieber found that, setting aside whether banks would be motivated to do so, banks simply may not be technically capable of that sort of monitoring and reporting. And according to one study, there are some 20 different ways that different banks — and sometimes the same bank — calculate whether someone has met the minimum balance threshold to receive a free checking account. That makes it hard for the average American to determine just what they need to do in order to keep their account free.

"They can more-or-less do it any way they want to. There are no rules about where the minimums have to be set, or if there are minimums how high they can go. It's a marketplace here. That's probably as it should be," Lieber said. "The tricky thing here is that when people do the math in different ways ... then it starts to get a little complicated."

To be fair, some banks do use a very simple technique for determining whether you meet the requirements for a free account. At those banks, if you fall below a certain number, like $500 or $1,000 or even $10,000, at any point during a month, then you'll be charged a fee. And while simple, that can be less beneficial for consumers.

"The more complicated way is they take all 31 days of the month, they take all 31 of your average balances on those days, then they add them together, divided by 31 and that's the answer," Lieber said. "And they don't do it until the end of the month."

Lieber said more banks are making it simple, but the largest banks, with hundreds or even thousands of branches, are the ones using complex systems.

"Other banks tend to follow their rules, or you just end up with more people in them, because they're bigger," Lieber said. "It's a tough task to make sure you're avoiding those $10 monthly fees."