Frames
Using frames in a website can simplify the web design work. Unfortunately, most search engine crawlers have trouble moving around a framed site and may fail in adding all your pages to the listing.

Sitemaps
Making a sitemap will make sure all the pages on your website are indexed by the search engines. If you own a dynamic website, you can use Google Sitemaps and XML to create a dynamic sitemap that will update along with the website itself.

Redirected Homepage
This is probably the most common mistake people make. All web server software has some way of telling which page is the default homepage for the site. If your homepage www.yourdomain.com redirects itself to www.yourdomain.com/anotherpage.html, then you need to change anotherpage.html to whatever file name your web server reads, most likely index.html or index.php. Why? Your domain name is what most people will link to from other websites. If your homepage isn't exactly that domain name, you're missing a lot of backward links that probably will boost your search engine rankings quite a bit.

Use META Tags for Duplicate Content
Many times search engines index same or similar products as duplicates, which results in the product page disappearing from the listing. To make your product unique from the other similar products out there, use the META tags to your advantage. Make sure you have a unique title, description, and keyword tag on every page of your site.