Like Words With Friends, make a speed game too where you can play within 24 hours and if you don't, you lose.
Just start lots of games. I normally have at least 15-20 games going on at all times. I check my games every half hour or so and someone, usually multiple people have worded me back by then.
You can do this, like just do that with your friend it won't say it in the game but you can do it. Say if you don't play by whatever you lose.