Search engine optimization is more than making your site rank well in the major search engines, it's about integrating it seamlessly into your page design without your visitors even being aware exists. SEO is an on going task that all webmasters need to undertake in order to keep reasonable positioning within search engines and recieve the subsequent traffic it brings. A common mistake many people make is to optimize their site once and believe that they can hold a rank forever.  This might be true for some obscure key words but there is so much information flowing around the internet that there will always be someone willing to take your site on for rankings.