The New England Journal has a helpful article on preventing malaria in patients taking short jaunts to foreign countries. Might be worth a look, as safaris in Kenya, Tanzania, etc, become more popular (as does exotic travel in general, though this may change given the economy.)
First step: Check here to see what the malaria risk is in the patient's destination country.
IDSA guidelines say that, for countries where there isn't resistance to the drug, chloroquine should be your first stop. Sadly, there's a lot of chloroquine resistance, esp. to falciparum malaria-- the most deadly kind. Choloroquine still works in Mexico, the Carribean, East Asia and parts of C. America and the Middle East.
For other areas, you should prescribe atovaquone-proguanil, mefloquine, or doxycycline. Your choice will depend on several things, like the patient's medical history, age and economic situation; this table in the NEJM article compares them and can help you suss out the best option.
And, of course, don't forget to tell your patients bound for malarial areas to wear insect repellent with DEET, long sleeves/pants and footwear, and use nets, screens or AC and closed windows when sleeping.