Vaccinations

The CDC website has vaccination recommendations, travel advisories, advice on food and water safety, insect protection, etc. To supplement that, I recommend that you begin looking into updating your vaccines as early as possible. Some vaccination courses such as Japanese Encephalitis and Rabies, require 3 shots each over the course of a month. Also, make sure to contact you health insurance provider in advance to find out if the shots will be covered because they are pricey.

Water

Of course, you can always go the bottled water, soda, or beer route. Common sense tells you anything you have to open with a bottle opener is safe to drink. However, in some places even "bottled water" may just be plastic bottles filled (or re-filled, as it may be) with tap water. We are taking along a Katadyn Hiker Pro Water Filtration kit. It doesn't take too long to pump the tap water through it and fill your nalgene or hydration sleeve for the day.

Food

I often get hypoglycemic if I haven't eaten recently. It's always a good idea to carry snacks (nuts, powerbars, etc) even if you don't have blood sugar issues. You probably don't want to be starving on the side of a road somewhere waiting for the next bus.

Bug Protection

Especially if you're traveling to malaria afflicted areas, you should treat your clothing with permethrin, as well as wear bug repelling spray or cream. You can also invest in clothing that comes pre-treated with insect repellant, such as Ex-Officio Buzz Off shirts, pants, etc. These are expensive but the repellent lasts much longer than spraying with a product such as Fite Bite which lasts about 2 weeks or 2 washings. Even in hot climes, long pants and sleeves are useful as well. In really hot places, like India, look into the clothes sold locally for clothing made of light colored, lightweight, comfortable fabrics, and be sure to spray around your ankles, where most malaria-carrying mosquitos feed.

Women

Tampons and maxipads can be bought everywhere, so there's no need to stock up on a years supply (which would take up your entire pack anyway!). You may not get the brand you're used to, but the general idea is the same. Some female travelers prefer using a reusable rubber insert for periods called a menstrual cup. See here for more details. For birth control, make sure to get your annual exam and enough supply of pills or patches to get you through the trip. To cut down on space, look into getting the shot, which lasts for three months. Getting someone to administer the shot abroad would be tricky depending on where you go.