Deet (N,N-diethyl-meta-toluamide) is the most effective and long-lasting insect-repellent ingredient available on the market, but it can have cardiovascular effects. Take care not to overuse products containing deet. Pediatricians recommend not applying it on infants, as it has been linked to seizures. Use deet in situations where mosquitoes are a health hazard, such as when West Nile Virus is a concern.
Physical barriers (clothing, netting) are a nontoxic first defense against insects. Some people swear by lemon thyme -- crush it to bring out the oils, then rub it on your skin. According to the Centers for Disease Control, oil of lemon eucalyptus can provide protection that is similar to low concentrations of deet.
There are also health-food-store insect-repellent brands. As with all botanicals, be mindful of allergic reactions: Do a "sniff" and skin test at home before heading out for your hike. If natural methods aren't keeping bugs at bay, stick to insect repellents that contain no more than 30 percent deet. According to Gina Solomon of the NRDC, higher-concentration products will not work any better -- they'll only be more toxic. When reapplying sunscreen, don't automatically slather on more deet; follow the manufacturer's recommendation instead.