Conventional wisdom maintains that the earlier you book, the better the price you'll get. But is that still true in the age of yield management and discounting on the Internet?

To find out, we devised four Thursday-to-Sunday weekend getaways—from Atlanta to Miami, Los Angeles to New York, Chicago to San Francisco, and Denver to Washington, D.C. Limiting ourselves to flights that left between 10 A.M. and 3 P.M. and centrally located luxury hotels, we checked airfares, hotel rates, and air-hotel packages beginning four weeks before departure—in all, 170 different searches at airline and hotel Web sites, as well as the major booking engines. Here are the results.

AIRLINES WHAT WE FOUND In all but one instance, advance planning secured the lowest fares, especially on major carriers, whose prices jumped dramatically closer to the departure date. With competition rising between discount and major airlines, though, prices did fluctuate. We saw a Los Angeles-to-New York JetBlue fare drop $110 two weeks before departure. In the example below, Delta responded to Airtran's rates one week before departure, slashing its fare by $260. ADVICE Book early. If you must gamble with late reservations, sign up for on-line fare-watcher services to learn when a price falls. Don't limit your searches to the booking engines; flights on discount carriers like Southwest Airlines and JetBlue are listed only on the companies' own Web sites.

HOTELS WHAT WE FOUND It's nearly impossible to predict when the rates will be lowest. At some properties we tried, such as the InterContinental and the St. Regis in Washington, D.C., the least expensive rates were available early. In San Francisco, however, room rates dropped as the departure date approached. For the most part, rates at the most expensive hotels we tested, such as the Mandarin Oriental, Miami, and the Ritz-Carlton New York, Central Park, held steady; these properties tend to offer fewer special discounts. ADVICE In such an unpredictable environment, it's best simply to reserve when you're ready. Be sure to check multiple sites, as prices can vary widely between the big booking sites and the hotels' own. Also, if the rates at your preferred hotels seem too high, wait a few days to see if they fall. One caveat: although booking at the eleventh hour can pay off (in the case of Pan Pacific, Expedia's rate dropped by more than $300), there's a chance that the lowest-priced rooms will sell out. If you are set on a certain property, this might not be a risk worth taking.

PACKAGING TRUTHS We also tested the much-touted air-hotel packaging features on the major booking sites to see what happened to their prices over time. We found the most significant savings at the last minute, usually about $200 less than if we'd booked components independently. Bear in mind that, if you're traveling to destinations served by low-cost carriers (whose prices are often not available as part of these packages), you might be better off purchasing your air and hotel separately.