I have put together a robots.txt file for eCommerce. This file has been specifically designed for Magento, but the principles remain the same across eCommerce sites. Before you just upload this to your site make sure you do you research.
User-agent: *
Disallow: /admin/
Disallow: /app/
Disallow: /catalog/category/view/
Disallow: /catalog/product/view/
Disallow: /catalogsearch/
Disallow: /checkout/
Disallow: /customer/
Disallow: /downloader/
Disallow: /errors/
Disallow: /lib/
Disallow: /review/
Disallow: /sendfriend/
Disallow: /skin/
Disallow: /report/
Disallow: /tag/
Disallow: /var/
Disallow: /wishlist/
Disallow: /*.js$
Disallow: /*.css$
Disallow: /*.php$
Disallow: /*?p=*&
Disallow: /*?SID=
Disallow: /pkginfo/
User-agent: *
Disallow: /admin/
Disallow: /app/
Disallow: /catalog/category/view/
Disallow: /catalog/product/view/
Disallow: /catalogsearch/
Disallow: /checkout/
Disallow: /customer/
Disallow: /downloader/
Disallow: /errors/
Disallow: /lib/
Disallow: /review/
Disallow: /sendfriend/
Disallow: /skin/
Disallow: /report/
Disallow: /tag/
Disallow: /var/
Disallow: /wishlist/
Disallow: /*.js$
Disallow: /*.css$
Disallow: /*.php$
Disallow: /*?p=*&
Disallow: /*?SID=
Disallow: /pkginfo/