Search Engine Optimisation is key to success, if you get all your business from search engines. Like rooter, said, there are other ways, like plain old paid advertising. There's good and bad SEO though. Making sure your site is best laid out and tagged for the crawlers is part of good e-business. You can push things a bit harder by spamdexing, but you always risk the search engines penalising you when they find a way, and it might not impress your customers. While advertising a product as "This product is nothing like Edison Chen Last Minute Make Money Face Cream" might get your site a good ranking, it might turn off some of your visitors.

It's always worth checking your sites against the search indexes, to see what ranking they have for different keywords. Then try optimising them (right words on every page, making sure words in images are repeated in titles etc). Trouble is, you have to wait a while for the change to take effect in the index.

I don't know about the sitemap so much, david. I recently wrote some software to generate a sitemap that was specific to a site, to try to reduce the number of redundant crawls that only differed in some unimportant URL argument. The search engines read the sitemap, and then crawl the same old redundant URLs! Google says on its site that they only use the sitemap as an aid, and it may not have a big effect on their crawl.