I'm just about to start doing a little work on www.yuptones.com that earns from the Cellware Affiliate programme - do you think i should add a blog or something to create searchable content and drive traffic?
A blog is a nice way to communicate with your clients about regular updates and changes that are to take place.
If you going to have time to update your blog regularly then yes add a blog. IF YOU WON'T HAVE TIME, then don't add a blog - it will just look bad if it is not regularly update.
Add a blog will greatly help your site. By adding more unique content,your SERP will be ranked high and thus it will help in driving traffic to your site. And this will increase your sales
Add a blog. And create some good content that is keyword rich. It will help you in driving traffic to your affiliate program.