Kinda tricky "getting in the mood" I've found - you feel like a right idiot standing in a dead quiet room with headphones on screaming to no one.
Make sure you've got some distance between the singer and the mic so you get a bit more body in the sound.
Sometimes too much foldback or hearing yourself in the headphone mix is a bad thing, like if you want the singer to actually scream, perhaps try not having ANY of his/her vocals in the headphones. See, if you're screaming and wearing headphones and your screaming is really loud right in your ears, you'll instinctively back off the volume so it's not killing your brain. But if you have to scream to hear yourself over the mix, you'll really try a lot harder and your screaming will be more natural (by which I mean less restrained). Depends on the type of screaming you're talking about I suppose, but give it a go recording with no vocals in the headphone playback.
See, in practice rooms, I bet you practice loud. Probably really loud - as you have to when competing with drums, and the vocalist is screaming over loud guitars, bass and everything else. It's easier to get caught up in the mood and be comfortable when everything is loud and sounds cool and all that. Your singer is probably quite used to singing live or in practice in that way...screaming to be heard, as it were. The studio is kinda the opposite, everything is dead quiet and it's not as intoxicating an environment, if you get me.
And have a few beers or something to loosen up and get in the zone... if that's your thing. Not too many! haha