I don’t think there is anything wrong with mental packing. I try to think of how many times I will need nice clothes. If I need more than one set, I"ll make sure at least I can wear the same shoes and belt. If I’m staying at a Courtyard or similar hotel, they’re likely to have a laundry so I can wash clothes there if necessary. If I’m staying at a center city high rise hotel, laundry will either be a pain, expensive, or both so I’ll pack more.
It only takes me a 30-45 minutes to pack, so I do it before going out the door.
When I say I don’t use any mental packing, I don’t mean I don’t think about what I will need. I just mean that I don’t see the need to start thinking about it days before the trip takes place, only just when I’m starting to pack.
If I’m travelling alone, I pack either before going to bed the night before, or just before leaving the house. Takes about 20 minutes, most of which is refilling pill cases.
If my wife and I are travelling together, she’ll have started packing on the weekend before we leave.
I have found that when I travel infrequently I tend to bring more crap than I do when I’m gone often.
That’s okay. When you need a corkscrew/mallet/can opener/zipties/bungee cord/rain poncho/headache medicine/tampon/dry socks/sunscreen/bugspray, you can come to my tent. I’ll have one. 
I start making a list a day or two in advance and pack the night/morning before depending on when I leave. If you pack too early you end up packing things you need.
Ha ha! I only see myself using about two of those things, but you are very generous to share.
Now seriously, folks, it’s clothing. Retrieve them from your closet, then put them in a bag. Or there are toiletries; go to your bathroom, get what you need, then put them in a bag. Or you need a mallet, rain coat, Swiss Army knife, steel drum set, Richie Rich inflatable raft, CB Radio, fishing rods, and two weeks worth of baby formula; gather them, then put them in your bag(s). Anyone care to explain to me why this is a week-long ordeal?
Pretty much right before I walk out the door. It doesn’t take me very long, and it’s rare that there’s something that I’d be at a loss without if I’d forgotten to pack it.
I should put a General Store sign in front of my tent, really I should. I’ve gotten very good at this: in the last 5 years, I’ve never not had something I need, nor have I had anything with me I don’t need.
Because my full checklist is three columns, one full page, in 10 point font. I’m camping with two kids and a grumpy husband for three weeks in a temperate zone where the temperature can go from 110 to 40 in a 24 hour period. I want my bases covered. I’m also packing with one of those kids, so I can’t spend too long at it at once or the four year old loses it. So yeah, about a week, here and there, puttering along.
But yes, a “normal” packing for trips like normal people take I can have packed up in an hour - assuming everything I want is clean when I start. Most of my mental packing time is spent figuring out the least number of garments I can bring to mix and match into the most outfits suitable for the most varied yet probable range of temperatures and activities. I hate overpacking, but I also hate not having something suitable.
If I’m flying, I try to pack the night before. I don’t want to be futzing around trying to find my black shoes. If we are driving, the laundry is usually done 30 minutes before we leave, and since I just washed a load of what I want to take, it all gets folded and put straight into the pack or suitcase.
I do keep a duplicate set of my toiletry stuff, in a ditty bag. So grab it, and I know I have soap, shampoo, toothbrush/paste, deoderant, etc.
Usually the day before, unless we’re going someplace just overnight and aren’t leaving until the afternoon. Most of our trips are incredibly varied as to temperature in the day/night, activities, appropriate dress, etc. So the actual putting shit in a suitcase thing is no biggie, but figuring out what’s going to cover hiking in the day and going to a nice dinner at night when it’s 30 degrees cooler, without bringing a ton of shit…that takes some preparation.
And there’s all the cords and accessories and shit you need for the cell phones and the ipods and the cameras and the laptops–you don’t normally take all that out the door with you, so it’s easier to forget them.
The main thing, though, is that I like to be able to get up, shower, step into the clothes that are already laid out, fill the water bowls, and walk out the door without any fuss or muss or looking for something that isn’t where it ought to be, or that is dirty when I thought it was clean, or any of the other horseshit that tends to crop up when I leave something to the last minute.
Seriously, starting to lay stuff out in dribs and drabs isn’t an ordeal. Oversleeping and running around like a maniac trying to make sure you have all your shit together with two dogs and a cat glued to your ass every step of the way is an ordeal.
I start a list anywhere from a week to a day before the trip (longer lead time gives me more of a chance to remember random things to add). I usually start packing the night before, and I finish the morning of.