How do you know whether your cost-per-click advertisement has been successful? The best way is to track your results and do the math to see whether you've made a profit.

Say your average selling price is $20, and half of that is profit. So every time you sell a unit, you make $10. That means you need to make sure that your total cost of clicks to sell that item is less than $10. Here's how to do it:

Make it easy on the buyer. Instead of having the link in your search engine ad go to your home page, have it go directly to the page of the product it advertises. That way the searcher will get instant gratification, instead of having to search through your Web site to find the item in your ad.

Monitor your clicks. At the end of every week or month, determine how many people clicked through from your ad to your Web site and how many actually purchased. This will tell you what the average conversion rate is -- what percentage of searchers can be expected to purchase in the future.

Compute your averages. If it costs you $2 each time somebody clicks on your ad -- that would be $2 CPC -- and you get one out of 10 people to buy, you're paying $20 ($2 x 10 clicks) to get $10 of profit ($10 x 1 sale), a great way to go out of business fast. But if you're paying $1 CPC and getting two out of every 10 people to buy, you're bringing in $20 in profit ($10 x 2 sales) for every $10 in cost ($1 x 10 clicks). You've got the system down.

Analyze, adjust, tweak. If your results are in the red, then you need to move fast -- by changing the amount you're bidding, making your key words more appropriate, changing your advertising copy, or even reconsidering your product salability. The key to fixing a losing advertisement is to make sure that you're not getting the wrong people, advertising the wrong product, or paying too much.