Site Code
In essence a good site is one that is written for human consumption, 'content is king'. Everything on a page should be relevant and visible.
It is well known that links to other pages within a site and links from other sites will help search engines to spider a web site. Some site coders are tempted to add more links than necessary in order to increase the number and variety of links.
It is mistake to make these invisible using CSS positioning or to colour the links the same as the back ground. The same goes for text, extra text should not be added simply to increase the key word density of a page unless it adds to the visible content of the page and makes sense. These are two well known techniques that will most likely cause your website to be removed from search engine listings - best avoided!
Increasingly it is best to code in accordance to the Index of the HTML 4 Elements. Techniques that are considered bad practice include 'keyword stuffing' this is where you code a site and add keywords in comment tabs or deliberately add invisible form elements or over use image alt tags. These are some of the grey areas, in essence you need to take care not to over step the mark.
Optimisation is not just about fine tuning a web site for search engines. Optimisation for search engines requires a good site located within a good network of sites. You need other sites to link to your site for search engines to believe the value of your site as well. Therefore, by conscously coding for humans and ensuring that your content is complete and appropriate you will hold the keys to optimisation success.