You need to evaluate where you are going. What is the environment you are going to be visiting. The insects you will experience will be very different on the windswept mountainside of the Presidentials in New Hampshire, vs. the open tundra of Denali National Park in Alaska.
If you are going to be visiting an area that is famous for biting insects, you need to come prepared. Remember, everything is tied together in a big circle of life. In the extreme northern climates the clouds of summertime mosquitoes feed the millions of birds that nest there. Without them, the birds would starve. In the deep south, the mosquito larvae feed countless insects, crustaceans, and fish. In temperate climates, biting insects peak in the spring and fall, once again to feed the thousands of migrating birds. Insects usually have a peak time in each area, be aware of these times is one of the first steps in not getting eaten alive.
Have you ever been in the forest and though that every mosquito within miles has descended upon you? You may be correct! Mosquitoes and other biting insects find their prey by sensing carbon dioxide. As air breathing mammals, we give off large amounts of carbon dioxide not just by breathing, but through our skin, and blood thirsty mosquitoes have been equipped to sense this release from miles away. This is where insect repellant comes in.
DEET, or N-diethyl-meta-toluamide is the active ingredient in most insect repellants, and comes in a variety of concentrations from as low as 4% to 100%. DEET masks our release of carbon dioxide which makes us harder to detect by biting insects. When we cover ourselves in repellant, we literally don't smell appealing to biting insects.
DEET based repellants come in liquid form, creams, lotions, pumps, and aerosol sprays. Generally anything below 20% DEET content should be avoided for anything more than a backyard barbeque. Conversely, high concentrations of DEET can also cause problems. In excess of 35% can cause rashes. Heavy use of concentrations in excess of 80% have been linked to short term schizophrenia, while behavior will return to normal after use is discontinued. Many people have noticed becoming irritable after extended use of high concentrations of DEET based repellants. Almost the entire population will find, that DEET based repellants are the best and safest choice of repellant they can use.
You may have heard about Avon's Skin So Soft. Avon's Skin So Soft is a skin care product that is used as a moisturizer. Many people swear by it as a mosquito repellant, and some non-scientific testing has indicated that it is pretty effective. Avon has never, and will never advertise Skin So Soft as an insect repellant. The lengthy and costly testing that goes into certifying a product is more than Avon wants to spend, and it moves away from the real features of the product. Avon Skin So Soft only seems to work against mosquitoes, and not other members of the biting hordes.
Some real backcountry folks use kerosene, which is what was used long before the petrochemical industry gave us DEET or other repellant products. Although kerosene is effective, it is damaging to clothing and skin, and is flammable. I sure wouldn't want to be around the campfire after slapping some kerosene around the back of the neck and on the arms to keep the bugs away!
If you have ever felt that you are the most appetizing item on the face of the earth to insects, or conversely insects don't seem to be interested in you at all, there is some truth to this also. Recently scientists have discovered that mosquitoes will select healthier people over sick people when selecting who to bite. They don't understand exactly why, but speculate that our chemical signature, what the insect "smells," is different depending on our condition. One does not have to be unhealthy to be lucky around flying insects.
When we swell up and itch from an insect bite it is because we are having an allergic reaction. The insects fluids are injected into our skin and our bodies treat it as an immune reaction, and the bodies extreme reaction causes the bump, itch, and the redness around an insect bite. A very lucky few, about 1% of the population do not have this reaction, and are not effected by insect bites. The body has either built a tolerance up for the chemicals injected in an insect bite, or does not recognize them as a threat.