Off-page SEO is the notion that factors external to your site have an influence on how your site ranks. So where do ethics come into the picture?
Off-page refers to backlinks - links on external sites which point to your site. Google counts these links as 'votes of confidence' for your site. It follows then that you should accumulate as many of these links as possible; this is where ethics comes into the picture.
Ethical link-building would involve building a site which has so much to offer that other external sites will naturally link to your site. The other side of the coin involves buying links which is very clearly in contravention of googles terms and more information about this can be found here
Ethics and link-building aside, you should understand your high-ranking competitors backlink profile. All else being equal, google is telling you that this profile of incoming links is a good one and you should therefore look to mimic this type of profile (in terms of relevance, quantitatively and qualitatively).
At Blackswan we use a variety of tools and techniques for mining and analysing backlink data to clearly understand what is required and how best to achieve the desired increase in rankings.