@vinil, this is a comprehensive list of SEO techniques and is really useful for those who are new to the SEO world. The three categories of On-Page, Off-Page and Healthy tips cover off most of the basic principles. It is very important to remember that when doing SEO for a website that all of these stages are used as you can do all the off site SEO you want but if the site is not optimised then a lot of this work will be in vain!
Also remember that SEO is an ongoing process......not a quick hit.
@Vinil, how are you finding optimising for Google rather than Yahoo! or Bing - it seems that you need to alter your tactics quite significantly between various search engines?
@tianamartin, keyword density is a contentious issue and with the various Google updates (Panda, Venice and Penguin) it is practices such as keyword density within text that seem to be falling out of practice. It would appear that much like how 'Meta Keywords' is no longer identified by Google as a valid SEO practice (as Vinil rightly pointed out), many SEO's have now switched to stating that keywords within content or text is following the same sort of path, and that there is no % keyword density to follow when optimising on-site. The majority of SEO's would now advise your content to be relevant, interesting and unique and through this practice you will get traffic. I personally might use a few keywords in there but only to reinforce what the page is about and make the content more relevant. HOWEVER, try to keep this minimal as it is very easy to overuse keywords and shuffle towards the more devious black hat side of keyword stuffing.
It might be easier to consider that a lot of people have realised how important search engine rankings are for business - compared with maybe early 2000's or '90's. As such, what might be viewed as 'easy' SEO techniques like Meta Keywords and Keywords within text are used by everybody and therefore are made void as SEO indicators by the ubiquity of users. If that makes sense? Just about anybody can write content repeating keywords - although this distracts from readability. More complex on site / off site optimisation techniques (as long as they are white hat) will give better, more sustainable results. If you want an article on various SEO techniques, have a look at this:
http://www.goup.co.uk/guides/
It is more of an extension of what Vinil posted and explains the various terms as sometimes these can be confusing.
I also read an extremely interesting forum on the topic of keywords yesterday although I am not sure that it is correct etiuette to post a link to a competing forum within this forum! Lets just say it is a very popular SEO forum and not hard to find if you want to.....
Hope this helps!