So if it's 5 degrees, dress for minus 5 degrees, etc. Dress as if you're going out to play in the snow. Long Johns, pyjamas or similar items worn under your regular clothes is a good start, and make sure you've got your warmest gloves, and a hat or balaclava. Like with houses, most heat loss is through the top of your head. So make sure to cover your head and ears.
Hot drinks are good too but avoid caffeine if possible.
I have always found my favourite soup to a very welcome drink in the small hours of the morning. Standing around gazing at the night sky you can make you get very tired and cramped. This could make your observing less than memorable and therefore more reluctant to go out again. This is especially true if the object or objects you are viewing are high in the sky as most meteors often are. This can cause neck strain because of the awkward and unusual position that you are holding your head in for long periods of time.