During World War II, Marines in the pacific found that mosquitos were just as deadly as the enemy. Carrying malaria in addition to their itchy secretions, the little beasties were impeding the war effort.
The war department spared no expense in a frantic effort to find repellants for mosquitoes and other disease-carrying insects. They tested tens of thousands of compounds, and they have never stopped searching and researching for a better insect repellant. In the 1940s, the answer was deet, or diethyl-m-toluamide, and it's still the answer today. Deet is simply the best mosquito repellent available.
Other bugs act and behave differently, but deet does not actually work the way most people think it works; that is, it doesn't actually 'repel' the bugs—it’s more like camouflage.
Mosquitos are attracted to warm, moist carbon dioxide, which we humans expel through our mouths and skin. Mosquitos have special receptors that are as useful to a bug as our eyes are to us, perhaps even more so. Keeping the analogy of our eyes, deet works like pepper spray in humans. With blinded CO2 receptors, they can't follow you. If they happen to bump into you, they might still bite you, but without being able to detect the t warm, moist CO2, they're not so sure you're edible, so they tend to ignore you.
It's not important to cover your whole body with deet-based repellant, but you do need to create a masking effect, to create a cloud, so to speak. Careful application in the right places is all you need. First though, there are a few bad things about deet. 1) It eats away plastic, so be careful around your watch, compass, iPod and other plastic gear. 2) It sometimes irritates skin, especially around the face. Use only enough deet to do the job, and avoid putting it on your face and hands. Too many times, deet from hands ends up in the nose or eyes, so it’s best to keep it off in the first place. A good start is to put a bit of it on your clothing, a squirt on the back of the neck, and a healthy dose on your legs. That seems to do the trick for many people. Getting it on your face or in your eyes doesn't really help hide you from the bugs, so it’s not really necessary.
There is a common myth that Avon’s “Skin So Soft” and other over the counter lotions and skin creams are the secret bug repellent used by “actual Marines”, but it’s hard to find anybody who will admit to this. As far as the Department of Defense is concerned, s nothing is as effective against mosquitoes as deet. Now, perhaps other bugs mayreact differently, but mosquitos are the chief concern because of their propensity to carry disease. If there was something better than deet, the Marines would be using it in an official capacity.
As far as brands go, Sawyer Broad Spectrum and Maxi-Deet are quite popular with the hiking community. They contain a higher percentage of deet than some other brands, like Deep Woods Off!, so you need less. It’s not a matter of science as to how much deet is enough for any particular person, and your experience may be different. Short of completely covering up, deet is the most effective mosquito repellent yet discovered, so if you hate getting bitten by these blood-sucking annoyances, deet can help.