I start laying aside things like toiletries and extra items we may need a few days before. That gives me time to go to the store and replenish any supplies we may be low on. Socks, extra underwear, books and shoes/sandals we're not using now but will be used on vacation go in at this time as well.
However, the actual clothes don't go in until the day of or the night before so they don't get wrinkled. About 24 hours before we leave, "the rule" goes into effect for last-minute packing and goes like this:
Anything personal that we've touched for the past 24 hours (deodorant, toothbrush, razors, etc) goes into a plastic bag after its last use and gets packed with the clothes.
I hate having a suitcase on the bed right before we leave because I like to run one last load of laundry and dishes, as well as change the bed, so I'll come back a clean house where I don't have to do anything but the laundry that was created while we were on vacation.