The only way to really guarantee top placement is to use pay-per-click advertising. (This is an advertising mechanism in which you pay for every click of a potential customer.) Most search engines offer this on a bidding basis, and if it generates more money than you spend on it, it can be sound business advise. In most cases, you would DO THIS LAST - after you optimize your web site and implement other promotional techniques. If you do an effective job with your website optimization and promotion, you may already come up top naturally and not have to use pay-per-click for several keywords. (This is expecially true for MSN or Yahoo.) Or if you choose to make a big (temporary) advertising campaign while you are optimizing (and have the money for this), then you may consider this.

Some keywords will cost more than others, depending on competition. Some will generate more paying customers than others. Do the math as you go along. Figure out the return on investment. This is seldom a one shot deal and often takes a few months of tweaking to get the optimum pay-per-click bid amount for the different keywords.

YOU WILL FIND SEO COMPANIES THAT GUARANTEE YOU TOP PLACEMENT FOR YOUR FAVORITE KEYWORDS. THIS IS HOW THEY DO IT. They use pay-per-click as part of the package and point to the pay-per-click listing, showing you your top placement. The natural links are normally much cheaper in the long run, since once you achieved a top ranking without pay-per-click you will have a steady stream of customers without having this kind of per-click fee.

Google is, of course, an exception. For the first year you will not likely rank for most of your desired keywords despite any valiant efforts on your part. During this period, I would encourage using Google's pay-per-click (AdWords). Although Google is the slowest to recognize your natural ranking - it will give the greatest return in the long run. While you do this - use the information they give you while you manage your pay-per-click keywords. See what is used most often - and what gives you the most sales (conversions). They have tools to help you with that.

ANY SEO FIRM that promises or guarantees top placement for competitive keywords is either selling you pay-per-click or is being boastful. Be wary with your money. You don't need an SEO expert to sign up for pay-per-click. You may use one to help you find the right keywords or perhaps to handle optimization techniques. If they are selling you a package, pay-per-click is often a justifiable part of it. However, don't let such a promise be a decision factor in selecting an SEO expert.