Logos- Consider inserting the logo of the sponsor when a branded post is made. This is a visual cue for readers that gets the “sponsored” point across immediately- as they say, “A picture is worth 1,00 words.”
Type- Make sure the type weight is heavier when you announce sponsorship and that you give it at least a slightly different treatment than any other common character styles on the site
Graphics- If the graphics accompanying the story are supplied by the sponsor make sure to attribute these clearly, consistently and transparently. Don’t think that tossing a tool tip on the picture that only shows who provided it when the user actively scrolls over it is enough.