How should one acknowledge assistance received via sites like SE?
The proper way to acknowledge any source other than your own brain is to formally cite that source in your bibliography; whether that source is a textbook, journal paper, proceedings paper, preprint, StackExchange questions, blog post, newspaper article, movie, cereal box, bathroom wall, or your mom is utterly immaterial.  If your source is a StackExchange site, then you must cite the StackExchange site.

See meta.math.SE, meta.cstheory.SE, and meta.SE for a discussion of when, whether, and how to cite StackExchange questions and answers. 

(Full disclosure: I have cited blogs, StackExchange questions, Usenet posts, newspaper articles, movies, and video games in my refereed journal papers, and at least one of my StackExchange questions has more citations than at least one of my research papers.)