Whether your web site is big or small there are five basic things that are must do's when it comes to search engine optimizing your website.
The first key thing is to take the time to do a careful search of all of your keywords and key phrases and make sure that they really do relate to what you are selling. Use a good program like Wordtracker or hire someone to do professional keyword research as not knowing your keywords is like driving blind in the dark.
Make sure you write a short descriptive title tag of what each page of your site is all about before you submit it to the search engines. This information can also help the users that are searching for you locate you easily. "Avoid the temptation of creating title tags that are longer than 30 characters maximum, since they might have a dilution effect in your rankings of certain search engines.
Make sure that the main text on your web page is the same as the keywords contained in your title tag. This is so that the search engines have something to hook into when they search your site. Try and repeat these keywords two to three times in the main body of the text without sounding repetitive it.
Add a sitemap to you r site as that assists both the search engines and users in assessing your pages. It also helps Google properly index your site.
Another tip is to make sure that your site is surrounded in some kind of link popularity program. Increasing the number of links that point to your site will boost you in the results pages. You should only link to sites that have similar themes to your site, and stay away from bad "neighborhoods" or from so-called link farms or FFA sites.