Tyler Cowen at Marginal Revolution has been writing about trying to survive in Europe around 1000 AD with a secondary question about other time periods, still in Europe. Asia, the Americas, and the Middle East have obviously come up - people have suggested, wisely in my view, that someone whose primary skills are academic might do better in the Middle East or East Asia around that time.
I find this particular question totally fascinating, but the discussion makes me scratch my head. First of all, there's a blind assumption that the traveler in question is male and white. Much discussion of how much taller and stronger than everyone you'd be (I doubt I'd be taller than most of the men), little discussion of the relative rape risk, assumption that the obstacles will be those faced by an ill-informed foreigner, not by a woman who may legally be property. Similar for race: if you look dramatically different from anyone the people there have met (not just taller/stronger/healthier/better teeth, but completely different skin color) you might have some interesting reactions.
Second, most of the advice is about how to take over the world. I find this kind of laughable, especially when people assume that some small piece of knowledge would be immediately influential, or that you'd be able to somehow leverage your vastly! superior! knowledge of marketing! or your basic understanding of something very technical like telescope lenses in order to revolutionize the society. My favorite might be the person who suggests posing as a wizard (which in and of itself might get you killed) by using 'a magnet, some wire, and a water wheel' to produce electricity (where will you get a magnet and some wire? also, educated people may already know about electricity: the Greeks did) or by using a lighter (basically a fancy flint and steel, which is a very old technology). Also foolish is the idea that a modern person with no fighting expertise or reputation would be able to convince the nobility that your military strategy would be better than theirs. Still, there are some pretty good ideas in this category: selling boiled water, working on camp sanitation for your local ruler's military (reduce disease casualties in wartime? very useful), bookkeeping, a few simple technologies.
All of them ignore the most important thing. I think most of us would find that we have a lot of potentially usable information, but it's not going to do you a damn bit of good if you get killed (as a witch, as a heretic, just by pissing someone off). Since the original question was about what to do to prepare, here's my advice. Start by learning wilderness survival stuff: shelter-building, basic navigation and time-telling, how to build simple snares, gutting small game, edible plants, fire-building (flint and steel is your best bet here - bowdrill is a royal pain). Most people of the time will probably be much better at it than you, but it'd be nice not to look like a total fool; also, I think people are likely to be the most dangerous condition you face, so being able to ditch people for a while would be pretty useful. Second, learn as much self-defense as possible. In both situations, don't concentrate on our stereotypical ideas about what knights and other nobility do (swords? not your friends): instead, learn knife and hand fighting for self-defense, focus on small game in hunting, etc.
Remember that people don't have the scientific method as an integral part of their culture yet, so even if you start doing something that improves health outcomes or whatever, they may not be able to fully notice or understand those improvements. That's a real liability if you're trying to develop a sanitation infrastructure or do medical work: it's not going to be 100% effective, and the first failure may call your entire work into question, depending on the explanatory model people are using. These kinds of cultural differences probably cannot be overstated, though of course human commonalities remain.
Personally, my only real skill is medicine. And not just boiling water or basic anatomy, where the benefits are not immediately obvious: because I have some wilderness medicine training, I'm pretty decent at treating sprains, breaks, cuts, and other day-to-day emergencies where the pay-off is fairly immediate and not just epidemiological. Depending on where I landed, that's probably the type of work I'd try for; if I were in Christian Europe, I might also head for a convent.