What strategies do you use to build backlinks to your inner pages? What types of links do you build? If I use inner urls on blog comments and article directories, wouldn't this look like spam?
Quite the opposite. If you have a page that contains information about "How To Use A Red Widget" on your "All About Widgets" site and you have a blog post you found on another site about "How To Use Widgets", it would be a better idea to link to your "How To Use A Red Widget" page rather than just linking to your main site URL. You are providing a link to information that would add value to the "How To Use Widgets" blog post for anyone who is reading that post and wants to know specifically about using Red Widgets. It is the same concept behind Google search results. You search for something and Google tries to give you the most relevant information in the search results. When you search for something specific, Google may return a site that has a broad subject, but you usually see it return a specific page that is highly related to what you searched for rather than just giving you the main URL for the site and leaving it up to you to navigate the site and find the specific page. When you provide a link directly to information that someone on another blog is likely to find relevant, it gives more value to the link (than if you just linked to your main site where the information may only be loosely related). That value means that someone is finding what they want so Google will like it, and there is a better chance that your traffic from that link converts and spends more time on page.
same way you build links to your main page. if you use auto commenting tools to post comments on pages that have few hundred spam comments already, then it might be considered as spam. but if you use proper blog commenting method where you find the blogs within your niche and write relevant comments, then it will not be spam. same goes with article directories where you submit relevant article to the right category and use the resource box to link to your main or inner page. most of the article directories allow 2 links in the resource box so you can actually link to your main page AND inner page form the same article
The links from other pages within your site also counts, so you should not neglect to optimize your site architecture, in order to supply more internal links to your inner pages. And try to shorten the click distance from home page to your internal page. For building external links to your inner page, it's just as same as you build links for your home pages.
No, it's not going to look like spam. Build backlinks to your inner pages just as you would your main domain.
I use SE Nuke and just set up my campaigns for the inner pages. Some of them end up getting to PR3 from it