In recent days, as you can see, I have been optimising Adsense blocks after reading a great book (download the book) that explains very well some basic and not so basic things about advertising. To begin with, I would like to tell you that although I have already read it, I have not finished it yet, this is due to one of the points that I will explain to you now. The order of the list is in order of importance in my opinion:
1º- Do your own testsand leave a week before making another change. (I'm still testing).
2º- The link must be in the colour of the links of your website (if it is blue, so much the better), the text in text colour and the text-coloured urls too. (This will make the ad look less like an ad).
3º- Choose text ads onlynot graphics. (I recommend testing with both types).
4º- No bordersand the background should also be the background colour of your website so that it is not noticeable. (If you put it, it will look like an advertisement from ten thousand kilometres away).
5º- Advertisements must be visible without scrolling(Most of the clicks on advertising are made without even moving around the page).
6º- Place advertising only for search engines, take care of your regular readers. (I'm working on it, hehe).
7º- The best block is the 336×280and the second best is the 300×250. (I also recommend testing this).
8º- The banner 468×60 does not give almost no click. (I also think it's a matter of trying it out).
9º- The order of importance when placing advertisements is as follows top to bottom and left to right.
10º- If your website or blog is about a very specific topic, I recommend you to use list of links in sidebars of your website.
11th- Usa tables for ordering the blocks you want to put together within a text.
All these ideas have been taken from my own experience and from a very good Adsense book (download book). As an additional recommendation, take a look at the Adsense application guide made by Smashin Magazine. Cheers and make lots of money with adsense!!!.