Many of the larger e-commerce stores edit their sites several times a week, and some do it every day. The first place to start promoting your web site is on general-purpose search engines. Don't expect your site to show up in search engines immediately. It will show up in AltaVista in a couple days, but most other search engines are slow to add new listings. Some only seem to rebuild their databases every couple months.
Most search engines filter out sites that try that.
The best approach is to use key words frequently in your site, but not in a way that appears unnatural. They vary a lot in quality. Some are, so far, just pages of links. However driving more traffic to a site does not always result in an increase in sales. Some companies have spent vast amounts of money of banner advertising without seeing a significant rise in sales figures.