The tattoo would depend on just what you do for the costume, I think. If you use some material that is going to rub and be restrictive and cause you to sweat a lot, then some sort of make up would be best. But if you are going to be staying cool, then just plain old magic marker would work fine too. Personally, I would recommend non-shiny, because fact is too many other cosplayers do her that way already, so it would stand out better not to go with regular materials. Also, I recommend something stretchy, but fitted. Let us bare in mind Lust is a cartoon character, and hence the laws of physics do not apply to her. But in real life, wearing a dress that is so damn tight and made of material that does not stretch is going to make it hard for you to do something as simple as sit down. Besides, the dress makes comparatively little difference, I think. Lust's character is all about attitude, baby!
Freedom is the right of all sentient beings-Optimus Prime