I think first aid kits should be customized for whatever situation they are most likely going to be needed for. For instance, if you're hunting you'll want things like Extra Absorbent Gauze, Compression Bandages, and Quick Clot for gun shot wounds. If you're out with the older folks you might want to bring along Aspirin or even Nitroglycerin Tabs (if you can get them) for possible heart attacks. Around kids? Make sure to have plenty of different size Bandaids, Hydrocortisone Cream or Xylocaine Cream for bug bites and poison ivy. In a hot environment, bring Instant Cold Packs (along with plenty of water) for heatstroke. Really, its all a matter of planning ahead, which I am a big fan of. I am fortunate enough to have access to lots of medical supplies and I generally carry a pretty extensive number of things in a first aid kit. I kind of over do it but that's me. Ultimately, the best way to render first aid is to know what you're doing. Get some basic first aid training like a CPR class or even an EMT class if you're really gung-ho about it.
A few things that I've actually seen come in handy for on the spot first aid are:
a Sawyer Venom Extractor http://www.sawyerproducts.com/B6B.htm
Duct Tape (no kidding)
an over the counter Asthma Inhaler
and of course, the one thing no first aid kit should be without, Vaseline.