Something to carry them in, naturally ...Plus my dice/counters, extra sleeves, newly acquired cards, yadda yadda.
I usually take two decks with me for FNM (unless I'm playing draft, then just one). I like having a spare deck for friendly games between matches so I don't get tired of playing the same deck all night. Plus it's a chance to test out new decks without making a tournament out of how much my new idea sucks in practice.