Seo secrets for sales landing pages ?

I have a landing page that I'm trying to rank higher in the search engines...(just like everybody...) Although I've built a tonne of back-links I believe that the sales page is lacking in keyword rich content. The sales page converts well and I think that any further content will just clutter up the page.

I have a heap of keyword rich content at my disposal and somehow want to attach it to the sales landing page so Google crawls these pages and boosts the sales page rank. But I don't want potential customers to have access to these pages via the links.

Is there a way I can put invisible links in the header image or something similar so that the sales page links to other pages of keyword rich material which would ultimately boost the rankings of the sales page. Of course the other pages would be on the same domain as the sales page.

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