Moving up to their last big house, moving over to a retirement home or moving out of the empty nest to smaller digs, baby boomers accounted for three out of four home-buying transactions in California in 2004.

Baby boomers—those born between 1946 and 1964 and now aged 40 to 58—took up much of the slack left by first-time home buyers who composed only 26 percent of the market in 2004, the lowest level ever, according to the California Association of Realtors.

In the process, many of them learned a new move—simultaneously buying one home and selling another.

It isn't easy.

In a hot market it can be a deal killer to include, in your offer to buy a new home, a contingency to sell your existing home. Sellers in high demand will simply take their pick of offers not burdened by such a contingency.

If you take a chance and sell your existing home without signing for a new one or being sure you've got one in the pipeline, you could wind up temporarily homeless or spending exorbitant fees on motel rooms or other high-priced temporary quarters.

You do, however, have several alternatives. The best buy-and-sell juggle is for those who can afford to own two homes—if only temporarily. Those who can afford this move may actually reap a financial return in the process.

Generally, right around the time school starts, Silicon Valley has a relatively slower market when a smaller buyer-to-listings ratio tends to stall prices or even force them down. The slower market often extends beyond the holidays and into the new year. In the spring, things begin to heat up after buyers have shaken off their sugar plum haze.

If you buy your new home when it's cooler outside, you likely can get more for your money. Then hold onto your existing home until the spring thaw or later, allow it to enjoy the faster appreciating market, and get more money for your home. Carry two mortgages in the interim and you'll benefit financially from the best of both worlds, according to Richard Calhoun, broker/owner of Creekside Realty in San Jose.

"Most people look at real estate in terms of a long term investment, but when it's time to buy and sell always look at the short-term financials. If you can accelerate or delay the decision to buy or sell, you can incur financial return," Calhoun said.

The effort also removes the contingency to sell your old home from your purchase contract.

For those who can't afford to own two homes, a more common move-up strategy is what's called a "rent-back."

In a rent-back, the seller sells the home but stays put, paying rent to the new buyer. The rent is typically the same as the new buyer's mortgage payment, including principle, interest, taxes and insurance.

The deal is useful when the seller is waiting for a newly built home or knows he or she is about to close or enter escrow on another home. Rent backs can be as long as 60 days, but the shorter the better.

"After 60 days it becomes non owner­occupied and lenders don't want to see that too long because it's an investment property," and subject to higher mortgage rates," said Mary Pope-Handy, an agent with Intero in Los Gatos.

The contracts can be an addendum to the sales contract, written like any other short-term or month-to-month rental contracts with a security deposit, walk-through property condition checklist and 30-day or shorter notice of cancellation—should the move-up buyer's new home become available sooner than expected.

The approach works best in hot markets, when inventories are low, or during peak buying periods.

"Sellers can ask for anything they want when they have five offers on their home," said Pope-Handy, also co-author of Get The Best Deal When Selling Your Home In Silicon Valley, (Gabriel Publications, $18.95).

The rent-back eliminates the need for a sale contingency, making the move-up seller's offer to buy in the best light.

Move-up buyers should be pre-approved for a guaranteed loan for their new home and if no specific home is in sight, should attempt to determine, in advance of signing a rent back, if a move-up home is attainable within the rent-back period.

A backup, but much riskier strategy is what's called a "bridge" or "swing" loan.

Using your existing home as collateral, you take out a bridge loan for three months to five years to use as the down payment on your new home. Once you've purchased your new home, you sell the old one and pay off the mortgage and the bridge loan. Such a loan is less risky in a fast appreciating market where appreciation can cover the extra payment on the old home. Even in the best market, however, swing loans can be expensive, last-ditch propositions that are fraught with caveats.

Typically funded with private money from investors, bridge loans can cost 5 percent to 10 percentage points more than a typical equity loan. Your home must be lien free. Excellent credit is mandatory, as are good income-to-debt ratios.

It may be a better idea to get a cash-out refinance, second mortgage or equity loan to use as a bridge loan. Traditional financing is cheaper and less risky, but there are costs associated with terminating a line of credit sooner than the typical term for such credit.

Real estate writer Broderick Perkins, executive editor of San Jose-based DeadlineNews.Com, writes regularly for this newspaper.