Financial Engines, which provides advice to participants of 445 mostly large plans, says that about a half-million plan participants with almost $44 billion in assets use its managed accounts, often those nearing retirement. Morningstar's Investment Management division, which offers advice to about 150,000 plans, many of them small, manages the accounts of about 746,000 people with about $19 billion in assets.

Click here to become a fan of MSN Money on Facebook

The service is most effective when it is truly customized. To get that, participants are asked to provide data about their investments outside the plan, such as other savings, old 401k plans or IRAs, and a spouse's earnings and retirement accounts. The problem is, most people don't provide all of that detail. And without it, "you're not going to get what you pay for," says Walton.

One-on-one help

If you want to manage your own account, you still may have the option of sitting down with an adviser or talking with someone on the phone who will consider your individual situation and help you create a plan. It will be up to you, however, to actually make the changes to your account and monitor it in the future.

TIAA-CREF, which provides plans to 15,000 institutions with 3.7 million participants, offers such counseling at no charge. It has 400 people based in local offices, an additional 200 who visit institutions where it offers plans and about 100 phone reps to provide such guidance.

People who take advantage of that one-on-one help are more likely to make positive changes in their savings or portfolios. Still, says James Nichols, who oversees TIAA-CREF's advice and planning, "one of the challenges is getting people to stay on track," especially as they age and their situations change.

Internet services

The widely available and free do-it-yourself service, where you plug your information into an online program offered by your plan and get recommendations back, is also the least used, according to a recent study (.pdf file) by Financial Engines and Aon Hewitt, which looked at how participants in eight plans fared between 2006 and 2010. More than twice as many participants in the plans used managed accounts as used online services.

The investors most likely to go online and put in the effort to get recommendations typically had higher earnings, saved a higher percentage of their pay and had larger balances than those who used managed accounts. They also tended to be a bit younger than the managed-account users.

The general lack of interest in taking advantage of easily accessible online services underscores how hard it is to get participants to think about and put some effort into their 401k investments.

"A large portion of participants are reluctant investors," says Christopher Jones, the chief investment officer at Financial Engines. Retirement investing "is down on the priority list -- people don't have the time for it, or the inclination."