Put your main keywords on the main page and then your secondary, longer keywords on the secondary pages.
Depends what you mean by main. I actually target keyphrases of similar weight on the front page as inner pages. All are very competitive. You could target real longtail on blogpost pages but for product pages you want to be competitive. Say you have a site selling Buckets. On the front page you might want to try for "Buckets", "Cheap Buckets" and "Large Buckets". On on inner product pages you want to hit things like "Square Buckets", "Round Buckets", "Blue Buckets" etc. That way you are hitting competitive terms throughout the site.
I would build your site and keyword structure as if it were a pyramid. Most important terms on page one, then break it down into a category structure that is logical. If you sell cars, have the main page be about cars, secondary pages being about brands and third level be about models that those brands make.