If you do not yet have a FAK, then buying one off the shelf is a good first step. But I suspect that you will find that you start to modify it pretty quickly based on experience.
Some of the FAKs I have bought have enough linen in them to truss all four legs of an elephant. None of them had 600mg Ibuprofen tabs for when you want to really forget about the pain. Ditto Betadine for washing cuts.
Into the space vacated by the yards of cloth, I include in my kit iodine tabs for water purification, one reel of fishing line and some hooks. Water-proof matches, a ferro-magnesium fire-stick, two Wet Fire tabs, a compass plus a signal mirror and whistle.
The rationale behind this is that whether I am day-packing or gearing up for a longer trip, I still know that the one (water-proof) little red bag holds all the essentials. It becomes second nature to chuck that one thing in the bag.
