What first aid techniques are most important for casual hikers?

The most important technique: ability to recognize common problems early.  These include recognizing: dehydration; salt deficiency; hypothermia; asthma; allergies; twisted ankles, and plenty more.  Note that I didn't even mention how to fix the problems -- people without formal training often try to memorize cures and first-aid techniques without focusing on the fundamentals of identification first, which is completely backwards.  Many problems, such as dehydration and salt deficiency, are easy to solve (e.g. drink water; take a hydration salt tablet; wrap the person in a space blanket; call for help right away) if you recognize the problem and have the appropriate tool to solve it available to you.  For the second part of your question, check out my first aid kit list here: This connects with the first part of my answer: know the most important circumstance to use everything in that list, and how to recognize it.

I agree with Rick's recommendations for either the Wilderness First Aid or Wilderness First Responder (WFR) courses.  The National Outdoor Leadership School (NOLS) runs both courses all over the U.S.  You can find more information on their website.

In climates with extremes temperature, you have to know how to identify and treat illness, like hyperthermia/hypothermia, frostbite and lack of fluids. For overnight trips, understand how to treat burns, as you will more likely to be cooking and/or creating a fire for warmth. Also, if you are hiking with another person or even group, make sure you know what medical issues they may have (i.e. diabetes, allergic reactions, etc.) and know how you can treat them if these people get sick. However, if you want the perfect the emergency first aid kit then you can consider the http://firstaidkit.us/ since the site is providing valuable first aid products. Thanks!!