Ethical SEO is to use seo techniques in ethical way and never use grey hat technique, cloaking, black hat technique, spamming, spun article and keyword stuffing. All the techniques used in quality and moderate quantity so that all the results appear are organic.
I believe ethical SEO is all about providing backlinks to your site from other relative sites and not spamming forums, other blogs, or simply sending out hundreds of automated article submissions to "stock" blogs that aren't relevant at all to your niche.
Ethical = legal. No buy links, fake pagerank etc. Just add links where you can add them, like directory, presell pages.
Ethical SEO strategy is nothing but going in a ethical/recommended way, either in off-page or on-page SEO. And i would suggest you to go through with the google webmaster quality guidelines, optimization tips for more details on the ethical ways.