(MONEY Magazine) – When you strike out on your own after years of working for someone else, retirement planning may not be top of mind. You have to set up an office, market yourself, find clients, buy insurance and attend to all the other pressing demands of running and growing a business. Saving money is an afterthought at best. Even if you can earmark some of your earnings for your future, you can't pick up the phone and reach your old corporate benefits department to set things up. No one, alas, is automatically subtracting a 401(k) contribution from your paycheck.

Now the good news: In an age of record self-employment--not to mention diminished corporate job security--you can find a do-it-yourself retirement plan that will fill the gaping void left by your missing 401(k). "There are good options, easy to set up and cheap to maintain," says Ed Slott, a Rockville Center, N.Y. IRA expert. And financial services firms, eager for a share of this growing market, are courting entrepreneurs with fee cuts and special services. Schwab, for example, recently dropped all account service fees on small business retirement plans.

The challenge, though, is figuring out which of the major lone-eagle plans best suits your business. What the SEP-IRA, Simple IRA and Solo 401(k) all have in common is up-front tax savings and tax-deferred growth: Your contributions are deductible, and you don't pay taxes on the earnings until you withdraw the money in retirement. There are, however, key differences among these major plans, most significantly how much you can put in and what happens when you add staff. Because you can't fully fund more than one in a single year, you need to make a choice. Read on to see which is the right plan for you.

Simplified Employee Pension IRA

BEST IF: You run a one-person show and plan to keep it that way. A SEP-IRA is the plan of choice for most sole proprietors and moonlighters. You can open one at virtually any bank, mutual fund company or brokerage firm, and pay low or no annual account fees. Your contribution limit is based on a simple formula: You can put away as much as 25% of your net income, up to a cap that is periodically raised to keep pace with inflation. But what's especially appealing is a SEP's funding flexibility. You can wait to fund the plan until you file your taxes. So if your income turns out to be higher than expected, you can make a large contribution and cut your tax bill. The opposite holds true if you have a tough year. "For self-employed people, SEPs are a great tax-management tool," says Keith Hall, a Dallas C.P.A.

Solo 401(k)

BEST IF: Your business is raking it in or you need to catch up with your savings. The self-employed version of the corporate benefit staple, a Solo 401(k) is an especially good choice if you are scrambling to build up your retirement savings and can afford to sock away a considerable portion of your earnings. The generous contribution formula lets you put aside more money at a lower income level than you can with a SEP. (In the unlikely event that you can save even more, you'll need a profit-sharing plan, an option that's expensive and complicated.)

It is also possible to take out a loan against a Solo 401(k), a great feature if you're running a business and worry that any money in a retirement plan will be out of reach for years to come. What if you need funds during a business crunch? You can borrow half the account's balance, up to $50,000, and typically take up to five years to pay it back (provider rules vary).

Unfortunately, Solo 401(k)s come with a bit of bureaucratic hassle. Once your balance exceeds $100,000, you have to fill out an IRS form every year, which will cost a few hundred dollars if an accountant does it (you certainly don't have time).

Simple IRA

BEST IF: You work alone but aspire to run a bigger business. What if you are having such a great year that you decide to hire even one full-time employee? With a SEP-IRA or Solo 401(k), you face hassles. SEP rules can lock you into expensive employee contributions. With a Solo 401(k), unless that helper is married to you, you'll have to stop funding your plan or convert it to the more complex and cumbersome employer version. For that, you'll surely need professional help, and you may even have to hire a third-party plan administrator, often at considerable expense. With a Simple IRA, you can keep investing in the same plan. Bear in mind that Simple stands for "savings incentive match plan for employees," so you have to match your employees' contributions, up to 3% of pay. The sole problem with a Simple IRA is that you can stash away no more than $10,000 a year ($12,500 if you're 50 or over), which may not be enough to fund a comfortable retirement.

Bonus: Add a Roth IRA

BEST IF: You've maxed out on fully deductible plans. While a Roth IRA isn't strictly a plan for the self-employed, it's a great option if you find that you can save even more after funding your SEP, Solo 401(k) or Simple IRA. You can put the full $4,000 maximum in a Roth in 2006 ($5,000 if you are 50 or older) if your adjusted gross income is below $110,000 (as a single person) or $160,000 (as a married couple). And doing so doesn't affect your contribution limits for other plans.

You get no tax deduction for your contributions. But the beauty of a Roth is that not only does your money grow tax-free, but withdrawals are also tax-free in retirement, giving you a hedge against higher tax rates later. "We may not see another '90s-style boom in our lifetime," says Mario Yngerto, a Dallas financial planner. "So you have to be smart about other things, like taxes. If you can create tax diversification in your retirement savings, it's a great move."

HOW THE PLANS STACK UP