If you live reasonably close to a "travel clinic," it would be worth the investment to go for a consultation well in advance of your trip.
Some conditions, such as typhoid, yellow fever and hepatitis A, are preventable by vaccines.
Antibiotics are useless unless they are matched to the most common diseases and drug resistance patterns in your areas of expected travel.
If you're using a custom insect repellent brew, test it on yourself before you travel to determine (1) that it works and (2) that you don't have an adverse reaction to it.
Finally, if you're travelling to the tropics, the most important issue is malaria prevention. You need to find out whether malaria is a problem at your destination, and what are the resistance patterns of the local strains. Malaria prophylaxis needs to be started before you depart and continued after your return. Detailed information is available on the CDC Malaria Map page.