Before you leave
Meet your doctor. If you have any medical condition like diabetes or hypertension, meet your doctor a week prior to your travel to ensure the right dosages of medications during your vacation.
Get yourself immunised. As different countries or regions have their own set of diseases, it's always advisable to get oneself immunised via vaccines or to take prophylactic medications before entering such zones. Make sure you have a tetanus booster dose. It's also advised to take a flu shot prior to visiting cooler destinations.
Pack your first aid kit and carry an extra pair of spectacles (if you have reading glasses).
Build up endurance. If your trip involves trekking or hiking, start building up your stamina. The last thing you want is to be stranded on your trek, however, beautiful the location may be.
Check your health insurance.

While travelling
Stay hydrated. Aeroplanes and trains are often packed, and there's also the issue of low humidity levels which makes nasal membranes dehydrated and more susceptible to infection from passing germs. Ensure you drink lots of water.
Move your calf during long flights; periodically raise and lower your heels while seated and walk up and down the aisle a few times. Sitting in a cramped position for a long time can form clots in the leg veins that can cause life-threatening conditions like pulmonary embolism.