Because I proctored a tactical medicine class at POLICE-TREXPO, I had the privilege of learning from some of the world's top military and tactical medics and trauma doctors. Even though I once worked as an EMT and have taught first-aid for many years, this class offered many nuggets worth sharing.
First, let's define an important term — the IFAK (Individual First Aid Kit). You must learn how it's been designed. You're not getting a traditional first-aid kit with bandages, ointment and tape. Many years of actual combat medicine and study have produced new thinking about what you should have with you at all times.
I won't tell you what to carry; ask your tactical medic for advice. Visit tactical medical Web sites as well as police and military vendor Web sites to look at the IFAKs and squad kits for sale. There are several different packed kits ready for purchase, and some require you to add only an item or two. You must be prepared to handle a "blow out" of bleeding. Tourniquets, hemostatic agents, and dressing should your main priority. In other words, prepare for self-aid or buddy aid for serious trauma survival.
Advanced emergency services do support us, and they can and have saved many lives. If you're in a crash — trapped and bleeding out — you will die before they reach you. You need aid in the first few critical minutes.
Do your research on which IFAK is best for you, and practice with it. I know most tactical officers, range masters and many defensive tactics instructors carry them on their person for immediate deployment. Having it in your locker or in the trunk is not acceptable. While in uniform, it's difficult to carry much more on the duty belt. A deputy colleague has one tucked in his jacket pocket, which is great for cool weather but summer is still not covered.
Don't expect your department to buy this for you. You're looking at a $30-$50 investment in survival. Yes, there are a lot of cool things you could drop these dollars on. I think living is pretty cool, so re-think this and get one.
When I said practice with it, I meant that. If you're a victim, you could bleed out at night or in low light; know how to apply care to yourself. This is not the time to read the instructions to figure out how this or that works. Your survival is what is at stake.
I carry a kit on my everyday carry bag with the other needful things of everyday life. I also have one in the console of the wife's sport utility (her Urban Assault Vehicle). Have it compact, marked and readily accessible under most conditions. I really don't want to apply it myself, but if the need arises, I'm prepared to continue the fight and face another day.