I know this is old, but hey, better late than never. As a couple folks mentioned, a darker background helps. James also had it right, putting the light pointing towards the camera through the steam. However, the key you’re looking for is directionality of the light. This means two things: the light is coming straight from the source and not splashing onto anything other than the steam, and the light source itself is not too large, washing itself out by shining from different angles. One can acheive that by using a gridded light, or just improvise one like that what I did here in 5 minutes. You could do this to a study lamp, or any other light source for that matter.
In this setup, the light source is just outside the frame to the right. sorry, it’s not a great, pretty picture as it was for illustration purposes only 😉
