I start with a commercially prepared first aid kit and then add/subtract as needed. What I've typically found in the FAKs advertising 150 items are about 100 bandaids that are suitable for razor cuts and nothing else. In a survival situation, shaving is pretty low on my priorities. Actually, unless I'm going to work or church, shaving is a low priority.

I weed through the bandaids ruthlessly, then upgrade the kit with more OTC medications, burn relief, trauma pads, gloves, etc.
The kits are a good start. Doug has some great recommendations for FAKs elsewhere on this site.