I start setting things aside several weeks before a trip. Certainly, if you are taking summer clothes out of drawers, you should put them somewhere handy if you think you may use them on your trip.
I am going the first week of Dec. and have just started my "last minute shopping" to buy items that I want to take with me: disposable bibs, a no-back car booster seat for DS, etc. I've got a box in the basement with items I put away after our last vacation: fanny packs, extra disposable placemats, etc. I'll be spending some time going through that box this week and making a shopping list.
A couple weeks before the trip, I open a suitcase and start throwing in items I won't need until the trip.
I don't use a packing check-list, but starting my packing ahead of time makes sure that I don't forget anything important.