I plan a traveling outfit, a day outfit, and a third outfit (evening, conference presentation, hiking, etc.) depending on the trip. I'll sometimes go so far as trying on everything I'll need to make the outfit work, including adjustments for hot, cold, or rainy weather.
If I know I'm going light, I then put everything on the bed and look for ways to cull down by looking for items that can do double duty, or can be worn in layers, or can be more easily laundered, or will add a bit more variety, or are more easily packed. Generally, I don't like to pack more than two pairs of shoes in a bag or more than one jacket. Unlike most people, I don't assume that I'll always be wearing bulkier items while traveling because I don't like to compromise comfort on a crowded plane or train, especially if the temperature gets warmer than expected, so everything has to fit reasonably well into either my handbag or bag.
My personal guidelines are
If I'm not positive I'll wear it, an item stays behind. If I guess wrong, I give myself permission to buy it if I decide I really need it. That scenario has happened only once when I decided I needed a pair of inexpensive sandals because of an unexpected invitation to a beach cottage--cost was about $10.
I pick the size of my bag, and set a weight limit (under 15 pounds)before packing. Everything has to fit into the bag and I eliminate stuff if I exceed my weight limit. I want to be able to easily carry my bag in a crowded subway car and up long flights of stairs without hurting myself.
No one knows if I'm wearing the same outfit uniform day after day so I don't expect to have the same choices as I do at home. Most of us can get by just fine on much less than we think we can. The less stuff I haul around, the more freedom I have to do things on the spur of the moment.
Quality pays off when it comes to traveling light and looking good. Since I'm wearing the same things again and again, I want everything to fit perfectly and look polished. I also get better treatment from service staff--like getting a complimentary pass to the executive lounge on the EuroStar.