Putting little or no money down on a home can be a smart move when home prices are going up.

But it's a bad idea when prices are going down -- and they're now declining in most of the country.

It doesn't matter whether you cover the entire purchase price with one big mortgage or a couple of piggyback loans. You could quickly find yourself upside-down, or owing more than your home is worth.

Suppose you paid $200,000 in an area where prices fall 10% over the next year? Your home would be worth only $180,000, but you'd still owe the bank more than $197,000.

That might not seem like a problem if you intend to live in the home five or 10 years -- plenty of time for the market to turn around -- but life doesn't come with guarantees.

If you lost your job or got transferred to another city, suffered through a divorce or major illness, you could be forced to sell.

You couldn't do that without making up the difference between the selling price and what you owe. Don't have an extra $10,000, $20,000 or more lying around? Then you're trapped in your home.

Before you take that risk, make sure you know how the median sales price in your town has changed over the last year and the latest trends in home prices.