Do affiliates sales decline over time?
You have a couple of things to consider here:
1) Increasing number of visitors means increased number of potential customers
2) Perhaps more importantly is to make sure you lever each visitor as much as possible. For example if you have 2000 uv per day and they never buy anything, then increasing traffic to 4000 uv per day might not help much.
It is worth making sure you get (2) right first, that way you have a fair idea of how you can increase revenue by increasing traffic.
Another factor is to "sell in context", whatever the subject mater of your site, try and sell related products. Web users rarely purchase something they arent actually looking for. With regard to Amazon, yeah, everyone knows all about them, but, someone might be looking for some info on a certain product - the content of your site - they find your site helpful and they are pleased, then you suggest they can get even more info from this book that can be bought from Amazon. Everyone is happy.
To ensure your affiliate sales dont fade away you must make sure you keep developing both of (1) and (2) above. Also, if you are providing specialist, interesting or up to date info then your site will aways be a good starting point regardless of whether the visitor knows of Amazon or not. If you can build a community atmosphere in your website then people will treat your website as their "first port of call", making you the first selling point as well.
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