My brother-in-law always packs late, on the night before the trip and probably gets no sleep! Don't do that.
I tend to make a "pile" of things I want to take and notes on things to do shortly after I book the trip. Sometimes that is a year ahead! Things like reading material, binoculars (cruise), neck pillow for the plane, a trading pin, notes on what I must see and do, etc.
For clothes, I have my outfits planned out a few months ahead and on hangers in a separate section of my closet. Then the day before I can just pull them out and pack them with the right amount of under garments. I make a list of what I'm wearing on what day and list any jewelry too. *list maker here!*
We have a toiletry bag that I restock with our favorite items a few weeks before. I also have a jewelry bag to bring a few pairs of earrings / necklaces. We mostly cruise so planning out our outfits is a must, due to the formal and semi-formal nights in the dining rooms.
One of our pieces of luggage has a garment bag section so that gets packed last so things don't get wrinkled. But it you have all of your clothes separated already that's a snap!
Being prepared a few weeks ahead of time will avoid the inevitable "wears my bathing suit?" ... or "hey, this doesn't fit" problems !!
So, it's never too soon to start packing. The more organized you are, the less likely you are to forget something and the more likely you will add some really great items to your luggage.
I also make a list of things to do before we leave the house on vacation.... like lock the windows, draw the shades a bit, turn down the heat/air, tell the neighbors, stop the mail, set timers on the lights, etc.