It's a good idea to visit your doctor at least four weeks before traveling to allow sufficient time for the vaccines to take effect.  Some generalizations governing vaccines for traveling:

Immune globulin can be substituted for the Hepatitis A vaccine.

Hepatitis B is highly recommended if you might be exposed to blood (e.g., health care workers), have sexual contact with the local population, or plan to stay for longer than six months.

Boosters can be used for DTP, measles, and polio.

A polio booster is suggested for countries outside of the Western Hemisphere.

The rabies vaccine is a good idea if you might be exposed to wild or domestic animals through your work or recreation.

Typhoid vaccine is particularly important if you are visiting developing countries.

Japanese Encephalitis vaccine is recommended for those staying for an extended period time, especially during the rainy season in rural areas.

Viral and Bacterial Vaccinations to Consider before Traveling