Joining and earning from affiliate programs is good, but if you don't know how your visitors move or use your site, then you probably or may make less than you'd like from affiliate programs. The reasons for this is because if you have a site that categorises things, then the affiliate programs or products listed on the first page will get far more traffic/clickthru's than the affiliate programs/products on the 2nd, 3rd, 4th pages, etc. I've looked at a site of mine and the pageviews that the 1st page gets is averaging 400% more than the 2nd page in that same category. Then the 3rd page gets even less and then less again for the 4th page, etc. In the end if you have 10 plus pages, the traffic dwindles off a huge amount. Obviously SEO can help increase traffic to the internal pages from each category, but a majority of your returning traffic will look at the first or 2nd page mainly and some may look at other pages to, but most probably look at the 1st page or 2. So putting your best earning affiliate programs products on the first page is a better option for getting it more traffic, exposure, etc.