When Chris Costello wanted to test his new online 401(k) advice service called blooom, he asked his sister if she would let him peek under the hood of her account.

What Costello found was typical of workers who do not pay much attention to their accounts - it was allocated badly, leaving her behind on her retirement goals.

In his sister's case, she had put her funds in a money market account when the recession hit in 2008 and never moved them back into the market.

"It's been like four or five years of recovery, and she had made like $10," says Costello, who is co-founder and chief executive of blooom.

Overall, workers have more than $4.3 trillion invested in 401(k) plans, according to the Investment Company Institute. Yet many of the 52 million workers who participate in 401(k) are not good at making their own investment choices, experts say.

Studies show that workers who get investment advice from any source do better than those who receive no advice.

The difference can be more than 3 percent a year on returns or up to 80 percent over 25 years, according to a recent study by benefits consultant Aon Hewitt and 401(k) advice service Financial Engines.

"Left to their own devices, people either do nothing at all or pick poorly," says Christopher Jones, chief investment officer at Financial Engines, the largest provider in the advice sector as ranked by assets under management.

So where can employees turn for guidance?

1. Start with your human resources department

You might already have access to advice, says Grant Easterbrook, an analyst who tracks online financial services for New York-based consulting firm Corporate Insight. He says even his own colleagues do not know they have access to free financial advice as an add-on benefit.

If you work at a big company, you might be one of the 600 clients of Financial Engines. Their free services include allocation advice and performance data. Other companies may employ consultants to give advice during open-enrollment periods or give access to calculators and other advice through the website of the 401(k) provider.

Employees at smaller companies might have to venture further to get help. "Three out of four participants don't have access to an employer-based advisory tool," says John Eaton, general manager of 401K GPS. "But there are a lot of DIY solutions out there."

2. Get free advice on the Web

The Web offers a lot more these days than standard retirement calculators. You can obtain detailed advice on allocating funds in your specific retirement plan from several providers.

At FutureAdvisor (www.futureadvisor.com/) and Kivalia (www.kivalia.com/), to name two, all you have to do is type in the name of your company and the system will generate a sample portfolio. You will then have to take that allocation advice and implement it on your own.

3. Pick managed funds or target-date funds

If you do not want to get too involved in the process - even to just pick a simple selection of index funds - your company will typically offer some kind of managed fund or target-date fund, a diversified fund linked to a future retirement date that gradually gets more conservative as you age, in their mix of choices.

When you allocate your money into these types of funds, you are buying the management expertise that comes with them, timed for a retirement date in the future. Sometimes that comes with stiff fees, so be sure to check the fine print, says Easterbrook.

"Absent engagement, it's a reasonable approach to take," adds Shane Bartling, a senior retirement consultant for benefit provider Towers Watson & Co.

4. Pay to have somebody manage it for you

Financial Engines has 800,000 subscribers who pay a percentage of their assets under management to monitor their 401(k) accounts and make changes accordingly. Others are GuidedChoice (www.guidedchoice.com/), which offers its services through providers such as ADP, Schwab, and Morningstar, which reaches 99,000 different plans.

Start-ups are emerging as well, either charging a flat fee such as $10 a month or a fee based on how much money you have.

401K GPS (www.401kgps.com/), which launched in 2011, operates primarily through investment advisers and small employers. There is also blooom, MyPlanIQ (www.myplaniq.com/), Co-Piloted (www.copiloted.com/) and Smart401k (www.smart401k.com/).

5. Do not opt out of auto-enrollment

The majority of people will still do nothing but that may a savvy option. Financial Engine's Jones says some companies are making workers re-enroll in 401(k) plans and defaulting them into managed accounts to get them to diversify.

"When we do that, about 60 percent of population will stay in these programs," says Jones. About 15 percent of active investors will opt out because they are already getting advice.