Point some quality links directly to the subpage. That gives the best results. That doesn't mean that you have to stop building homepage links. The linkbuilding process should be balanced (more natural for Google )
you build link to subpages which will not only boost your inner page PR but also your overall ranking as your subpage will then boost your main page as well
yes... not likely u see a strong subpage but weak main page. typically, main page should go along or higher than subpage.
Treat each subpage as it's own website, the best way to build links is writing articles and submitting them to article directories and getting involved in different social networking sites like propeller, facebook, myspace, twitter, digg, etc.
Why is article writing the best way? If i submit a article to 50 article directory's i will have 50x duplicate content how can this be a good thing? I do agree on the social bookmark sites. And try to get some high pr incomming links.