Here's a part of your plan that's easy to overlook. I'm very thankful that I haven't had to use most of the stuff on this list, but I always appreciated having it around. If you're going anywhere outside of major cities in the developed world, this list is for you.
Obligatory Disclaimer: I am not a medical professional, and nothing here should be considered a replacement for advice from your doctor.
Many people will go to their local REI or even Wal-mart, and buy a first-aid kit, if they decide to carry anything at all. These pre-made kits are not going to cover even a fraction of your real needs on road; having five different kinds of Band-Aids isn't going to help with traveler's diarrhea. Indeed, before you can make your own travel medicine kit, the first thing that you need is knowledge of what you're up against.
The government-funded Centers for Disease Control and Prevention maintains some amazing travel health resources. In addition to all of the obvious stuff on this page, be sure to check out the online edition of the Yellow Book, perhaps the most comprehensive and thrilling guide to obscure infectious diseases in the world. I find this stuff fascinating.
Still, reading the Yellow Book can make someone think that the world is full of bizarre pathogens ready to kill you at a moment's notice. The reality is that most of these diseases are rare, avoidable, or treatable. There's only two prescription medications most travelers should absolutely carry: anti-malarials (if you're in a malarious area), and a broad-spectrum antibiotic like Cipro, for serious, prolonged traveler's diarrhea. You'll need to talk to a doctor about the appropriateness of both of these.
Other than that, I carry a few tablets of Benadryl (diphenhydramine) to counter dangerous allergic reactions, and some Tylenol (acetaminophen, paracetamol) for use as a fever reducer. My doctor tipped me off that the acid reducer Tagamet (cimetidine) can be of use for allergic reactions that aren't entirely stopped by diphenhydramine. Most important can easily be found in most parts of the world, if you know their generic names, so you don't need carry more than a few of each to address urgent problems. Rather than carry around big bottles of these, pick up some of these little keychain pill holders, and use them instead.
After a year on the road, I'd only used one of these meds: paracetamol, once, while suffering from dengue fever in Thailand. Far more important to my recovery from Dengue, however, were the oral rehydration salts that I used to recover from diarrhea suffered during dengue fever. Ceralyte is the brand I carried, and I've since been informed that they have the best formula.
I can't emphasize enough how important rehydration salts are. Diarrhea is the most common ailment among travelers, and you don't appreciate how dangerously close you are to dehydration until after you've made yourself drink a packet of this stuff. (It doesn't taste great). Sports drinks like Gatorade are no substitute.
This is far from a complete list. There's quite a bit of hardware that I carry with me as well, which I'll take up in another post. In the future, I'll also write about some of the intangible things that I carry with me, like travel medical insurance and emergency evacuation services.