“I’d give up if I were you, you’re flogging a gift horse”
Origin: flogging a dead horse + look a gift horse in the mouth
“Flogging a dead horse” has many wonderful variations, let us know which you have heard or say…they don’t have to be mixed could just be an evolution.
Metaphor Madness BETA 
I actually though ‘flogging a dead horse’, meant trying to ’sell a dead horse’.
“Beating a dead horse”
http://www.escapistmagazine.com/videos/view/zero-punctuation/84-Console-Rundown
I always through this was correct because I’ve heard a lot of people use it, but iamdawn insists it’s “flogging” :P
There’s probably something in here about “Beware of Greeks flogging a dead horse in the mouth”.