How to build a rainy-day fund you can maintain

In the Greek myth, Sisyphus was condemned for eternity to roll a boulder up a hill, just to see it roll right back down.

For those of us trying to build up emergency funds of short-term savings, it can feel like a similar exercise. As soon as you manage to put some cash aside, you have to draw it down for sudden expenses — and then start all over again.

Natalie Smigielski knows the feeling all too well. About a year and a half ago, the 33-year-old from Toronto had scrimped and saved enough to come up with a $5,000 emergency fund.

But then Smigielski was hit with unexpected bills for car repairs, around $2,500. Then she spent a little more on Christmas gifts for friends and relatives.

She quickly found that her fund was down to a few hundred bucks.

"I had all of this money, and all of a sudden it was almost gone," says Smigielski, a government employee. "I was like, 'Wow.' It made me feel so insecure. What if another emergency happened on top of that?"

It is a common worry. Building an emergency fund was identified as the No. 1 financial goal of millennial women, according to an exclusive analysis provided to Reuters by Boston-based money managers Fidelity Investments.

"Since half of Americans say they would have trouble even coming up with $1,000 in an emergency, this was great news to hear," says Meghan Murphy, director of thought leadership for Fidelity, which polled more than 7,000 401(k)-eligible workers.

The best amount to save, according to most planners: at least three to six months' worth of living expenses, which should be sufficient to weather rough financial moments.

And if you spend it? Planners say that is what it is there for.

Here are some tips for how to keep pushing toward your goals:

If you keep your emergency fund in your regular checking or savings account, it might be too tempting to use. Create a separate account linked to your other accounts, so you can transfer funds if needed.

Treat your emergency fund as a monthly bill and automate deductions from your account. Much like 401(k) savings, if you do not sock the cash away before you even see it, then your emergency fund likely will be dead in the water.

Do not give yourself any wiggle room. An unforeseen medical bill or a broken-down car or a sputtering furnace are legitimate emergencies; a sale at Macy's or the latest smartphone upgrade are not.

If you are forced to draw down your reserves, use gifts, raises or windfalls to refill the coffers. Now is actually an ideal time of year to do that because of tax refunds. The average IRS refund last year was over $2,800.