I usually start with a list on my computer...things to go in the Carry-On bag, then things for each suitcase. Mostly I do it because I have a toddler & its so easy to get caught up in regular day-to-day stuff & completely forget something important (and some people say I have OCD! lol) The lists really help & I put a mark beside each item that can't be packed until the last minute (i.e. hairdryer, toothbrush, hairbrush, makeup, etc) so I will remember to throw it into my bag either the night before or day of our trip.
I usually throw things in the suitcases as I think of them (usually this starts about 2-3 months before our trip) and then 3-4 days before the trip, I take my list & lock myself into a room by myself (so I am not disturbed by munchkins unpacking as I pack my suitcases!) At that time (3-4 days prior) I make sure everything is in the suitcases. I also photograph what I am putting into the suitcases in case of a mishap with the airlines losing luggage. (I figure they can't argue value of items with me if I have both a list & a photograph!) Everyone who sees my lists things I'm half nuts...but its DISNEY...my lists are also broken into sections (already have, need to buy, will use everyday, take just in case, etc.) My lists may be a little overkill, but I do suggest having them especially if you pack waaay ahead of time. This way, you can look at the list & see whats already checked off...then you know you already packed those items or don't need to purchase them!
I just realized how OCD I sound...
Anyway, we aren't going on our trip until late November this year & I already have a carry-on bag half packed! I just pick things up when I see them or if I have some extra $$$ so I don't have to spend a lot all at once. Since we're going in Nov/Dec, I will be hitting the end of summer sales to get some things for our trip as well!