Pop-up modal screens are (by their very nature) disrupting a user experience that is already underway. So you have to work extra hard to grab a user's attention.
First, you have to ask yourself some important questions: "who is the audience and what are they in the middle of doing when this pop-up screen appears?"
How relevant the interstitial message is to both of the above will determine how hard you're going to have to work.
The headline should capture as much of the product as possible, but it's also gotta be short and sweet so as not to discourage the user.
The body copy should emphasize the secondary value propositions of the product—whatever they are.
Convenience? Variety? Quality? Rarity? Focus on the unique value propositions relating to the product that are easily understood at a glance. It also never hurts to inject a bit of loss aversion into a modal experience.
Now, this is an extreme example, but if you want to discourage a user from 'x'ing" out of your experience, just introduce the idea that they'll never see something again.
Even if it's a crappy product, something about evolutionary biology has hardwired our brains to need our options to be kept "open."
To be clear, this is a slippery strategy. But we're not here to judge.