I often don't even bother counting things like lettuce, because of how many cups it would be to total a block of carb. The rest is fairly easy... you don't need to mash things down, but you don't need to fluff them up either. For example, I chop up my broccoli into pieces that I would eat them and then pile them in my 4 Cup measuring cup to see how much it is.
Green veggies aren't really going to matter all that much, since even if you are a cup over, it adds up to a negligible amount. It's more important be accurate with fat, protein and high density or unfavourable carbs.