How should I credit co-authors of a paper in conference proceedings I prepared myself?
The practice I'm most familiar with is that a coauthor is a coauthor is a coauthor.  Anyone listed as a coauthor on the title page of the paper should be listed on the title slide of any talk about the paper, and explicitly acknowledged ("This is joint work with...") when the talk begins.  Whether any of your coauthors helped you prepare your slides or the proceedings abstract is none of the audience's business.

But this attitude is clearly specific to my field, where papers only rarely have more than three authors.  It might not scale so well in fields (like particle physics?) where a "small" group may have 100 members.