It'll never look 100% convincing, but honestly, who is really capable of getting those airbrushed, magazine quality, dehydrated abdominal anyways? However, this would be more convincing with a little
bit of natural definition.
A simple search on youtube resulted in multiple videos on faking abdominal muscles using basic makeup. If its intended for all day wear and you may potentially sweat, you may want to consider investing in proper theatrical makeup, like kryolan aquacolor or ben nye and an appropriate sealant.
<---- IMO the best one
Essentially, I suppose the trick is subtlety, and to have some pre-existing tone to contour. The one I labelled as what I perceived to be the most informative would also likely be useful for cosplaying male characters as well as female characters. Also, it'll probably look better in natural light photos than in flash photography.