When you're shopping online, the price you see isn't necessarily the best price. I'll tell you how to land the best deal.

Airlines have long used something called dynamic demand pricing (aka yield management) to determine the price of seats on flights. Historical analysis of sales, behavioral patterns of buyers, the rate of bookings and, even unemployment rates, among other factors, control the price per seat. No two seats are sold for the same price.

Now that idea has been spreading to other consumer segments, particularly online selling.

A recent Wall Street Journal analysis of prices on a high-end microwave was particularly revealing. The pricing changed nine times in a single day from $899 to $744. Not on a similar item -- on the same exact item.

Even within an hour, price can change three times from the same online seller!

So the price is not necessarily the price. Most people won't obsess to the degree of checking prices nine times a day. At the most, they'll just comparison shop once among two or more websites.

But if you really want the best deal, you must check not just once or twice, but repeatedly. If you feel your time is worth more than the potential savings, that's fine. Don't do it. But knowing that prices never stop moving is very important if you're looking for the absolute lowest price.

If you want an easy way to see if a quoted price is a deal or not, you can compare prices on websites like Decide.com or ShopoBot.com, or use a browser bookmarklet such as Hukkster.

Another alternative would be to install a browser plug-in like Invisible Hand that automatically pops up an alert while you're shopping if a better price is available on another website.

Finally, Amazon customers can typically get a better deal if they put something in their cart and then abandon it before the final purchase. That usually signals to Amazon that you're willling to walk away and triggers a lower price the next time you put it in your cart to checkout. Give it a try!