I like to use Benjamin Moore's M-83 oil and grease emulsifier. Have used it a number of times in the past and its never let me down. Of course there are plenty of other products out there to try, but I don't have much experience with them.
Just make sure to "profile" the concrete. If it's slick, and you don't create a tooth for the epoxy it will surely fail. Acid etching is ok, best is shot blasting.