Drink water or low-cal drinks. Sip it slowly over time if you're watching tv, etc. The hand to mouth action may help with the urge to snack, which is sometimes no more than the habit of hand to mouth (similar with some smokers trying to quit).
Look at the people who are snacking and grubbing. Are they as healthy as they should be? I would use them and their eating habits as examples of what not to do. Might help to keep you on track.
If they're healthy and fit and able to snack at night, I guess they're really active and burn it all off that way - in that case, you can still use them as an example of how exercise burns calories :)
What you eat can have a direct effect on your skin if you're struggling with psoriasis. See what to shop for.