Bearing in mind that high demand phrases are going to be very difficult to rank well for in the search engines, you need a strategy that allows you to get traffic to your "high demand" pages without having to rank well for those phrases.

Here is a strategy that I use on my sites:

1. Write articles, each targeting several low competition, highly-related phrases. These "themed" pages often rank well for multiple low-competition phrases. Even though each phrase is usually low in demand, the combined demand of all phrases on the page add up to significant traffic per page.

2. Create main pages that pre-sell affiliate products. There can be one or more main pages, depending on the niche. These main pages usually target very high competition phrases because of the nature of these pages. It is difficult to rank well initially for phrases targeted on the main pages.

3. Funnel the traffic from the articles, to the main pages, where we have a chance of getting an affiliate sale.

Using this strategy, you can get visitors to your main pages, without having to rank well in the search engines for the phrases the main pages target. Your articles will get the traffic by ranking well for several low competition phrases, and deliver that traffic to the "sales" pages.

As time goes by, and you add more pages to your site, and work on the off-page factors, your main pages will climb in the rankings, and eventually rank well themselves, giving your site a massive traffic boost.