There is a big controversy over what types of gear you should stow in your first aid kit. Some people believe you should stock an entire operating room, where others believe all you need are a few bandages and some pain relievers. Each side of the aisle will make valid points for each method, and that is okay. I personally believe that your personal first aid kit should contain enough stuff for you to help yourself in an emergency. Carrying around a bunch of surgical equipment will do you no good if you cannot get easy access to the wound. Plus, carrying an excessively heavy first aid kit will be cumbersome. First aid kits take up a lot of space in your pack. True, they are quite possibly the most important items to have... right next to water... but like water, you don't want to carry so much that you drop dead from exhaustion.
Below is a list of the things I have in my light and fast first aid kit. Note, this kit is my personal kit for backpacking. It's not a kit designed for anyone else to use because I have tailor-made this kit to my specifications. Yours may be different, but this will get you pointed in the right direction:
10 adhesive bandages (regular band-aids)
2 adhesive pads, 3"x3"
3 sterile gauze pads, 3"x3"
4 butterfly bandages
1 roll of adhesive medical tape
1 roll of cohesive bandage wrap (better than elastic)
1 moleskin pad, 3"x6"
1 small bar of soap or small pack of cleansing (alcohol) pads
1 tube of antiseptic (I used Neosporin w/ pain reliever)
3 small packets of basic first aid creme
3 small packets of burn gel
1 pair of Scissors
2 pairs of non-latex gloves
1 mouth/protection barrier for CPR
1 pencil and paper
1 small baggy of medications I take
Iodine and neutralizer tablets (for treating questionable water)
This kit fits into a small bag that is then stuffed into the bare bones bag (will be covered later), which will never leave my side in an emergency situation. In my backpacking pack, this kit resides in the top compartment, easily accessed in an emergency.
Your first aid kit should be the last thing you pack into your bag. The reason is because if you need it, it will be the first thing you see when you open the flap. Other bags, which have pockets on the outside, should have the first aid kit located toward the center pocket, where it will be protected from impacts on the sides (say, from brushing up against stuff). It also makes it easy to tell someone else, "Hey, in the middle pocket, there is a first aid kit..." instead of saying, "...the one on my right... no your left, yeah, the one on your left... no I meant right." Basically, you want your first aid kit easily accessible and protected from impacts.
If you can get your hands on anti-biotics, it might not be a bad idea to put them in your kit either. However, I have been unsuccessful at finding these because they are normally kept under lock and key at hospitals and clinics.
Remember, many of the items in a first aid kit do have a shelf life - especially medications and pain relievers. I would not trust a band-aid that is over 5 years old either. It would be best to rotate out your first aid stock with the one you use at home. Since you are most likely to use the first aid kit at your house, you should place your aging equipment into your home kit, and place the newer supply in your personal kit. This is not to say that you are placing old, outdated, or defective gear into your home kit. It's more like placing a new can of beans in the back of your pantry so you eat the old one first, before it expires. Otherwise, you will end up with old consumables that may or may not work when you need them to.
Rotate your medicines. I keep Ibuprofen in my first aid kit because not only is it a good pain reliever, but it also a muscle relaxer, which does wonders after a long day out hiking when your back, shoulder, and leg muscles are just aching. It is also that quality that makes Ibuprofen an anti-inflammatory medicine. I have a condition known as Sciatica, which causes the Sciatic nerve to flare up on occasion, especially when the muscles around the nerve are put under great strain. My doctor recommended the necessary dose for me to take to get the inflammation down when this happens. Therefore, I have plenty of Ibuprofen in my kit. It doesn't take up a lot of space, and pound for pound, it is quite possibly the most valuable consumable I carry. Other medicines you may need should also be organized and rotated frequently.
Also, remember to put a desiccant pack into your first aid kit. Moisture is the enemy of almost everything in there!
I also recently added Iodine and Neutralizer tablets to my first aid kit. The small bottles fit in there without a problem and also requires me to open the first aid kit to get to them. It is important to not let your kit just go unopened forever. I've seen what happens when people never crack open their kits. They assume something is there, only to find out that it isn't.
This brings me to my next point, and I want to reiterate the importance of customizing your kit to you. Don't buy the massive kit that has 144 million things in it that you won't use. You should keep a kit like that in your house. What I did was buy a basic $8.00 kit and added the stuff that I wanted. It's not the cheapest way to go, but in the end, it gives you the most bang for your buck. The only things in there are things that you may need and none of the things you don't.