Google Adwords - Keyword Tool
It is important to make sure that you optimize your affiliate site for your chosen keyword. You need to make sure that the URL of your site contains your keyword or keyphrase. This will be important in getting your site indexed by google, ranked highly in google and building traffic.It is best to use Google Keyword Tool before you create your affiliate site. Use the tool to search for keywords that relate to the topic of your site or the product you are going to promote. For example, if I was promoting 'iPhone accessories' I would enter this as a keyword phrase into the google keyword tool. The tool will then generate a long list of possible keywords from this phrase and give you information on the number of searches it gets and the level of competition. The competition related to the other number of sites with this keyword compared with the number of searches the keyword recieves.
You want to pick a keyword that balances competition with number of searches. This is create the best possible chance that you will be able to rank highly in google for the keyword and still gain traffic. Once you have your chosen keyword you need to intergrate it into your site URL and naturally throughout your content. The more times you use the keyword within your content the better.
If you optimize your keywords and create at least a few backlinks with your keyword in the link title and anchor you can very easily rank highly on google for your keyword and even get onto the first page. Make sure to ping blogs or lenses whenever you update them. Ping them here: Blog Ping for Squidoo Lenses or Ping-o-Matic
Google Keyword Tool: Check it out!