I probably start packing about a week before going somewhere. the week before that week, I mentally plan out what clothing I want to bring so I can make sure they are washed before I pack them and that the clothes I'll be wearing before I leave aren't going on the trip.
I also pile up stuff the week leading into departure. When I actually pack, I'll change my mind about some items and switch things out, but having that pile is just a reminder.
As related to toiletries, I keep a toiletry bag with small bottles of what I use. Sometimes they are empty, but seeing the bottle in my toiletry bag reminds me to fill up the small bottle or buy a replacement. If I know that I'll be staying a hotel that will have free to toiletries, I take only the items that they hotel won't have. If it's a longer trip where I might need nail polish remover, I bring a small bottle with me, but otherwise leave it at home.
Oh, it also depends where I'm going. If it's somewhere in the US, I don't worry about forgetting stuff, as I'm pretty sure I can find what I need somewhere nearby.
However, I just went to Hong Kong on business - just last week. Between knowing that I wouldn't have time to shop, knowing that sizing of clothing is different, knowing that the familiar chain stores would not be readily located and knowing that goods are much more expensive there, I made sure I was loaded up with OTC medicine and other personal care items .
Yeah we've hit the 15 day mark until I need to be packed and ready to go. I am starting to pull all of my travel stuff together and get the packing list going. It's going to creep up on me fast if I don't start paying attention.