I always take a ton of paper -- I've never figured out how to do that kit thing. I leave home the stickles and embossing powders -- too messy to use at a crop.
And I hope Rebecca (byondbzr) doesn't read this -- I also leave home the paints. Not enough time to dry. I really like to take my most frequently used items.
I get a ziploc for my ribbons. I don't take full spools. I keep a bag of cut ribbon (about 18 inches) to take.
Look at your completed pages -- take what you use the most frequently. If there's an item you used 2 or 3 times a year -- Leave it home.