Most repellent lotions applied to the skin contain a chemical called DEET which has been used since the 1950's. DEET is a very effective chemical good for areas with a lot of mosquitoes or areas where they carry serious disease.
If your in an area with only a mild mosquito problem or just want to avoid DEET then there are several other "natural" options to repelling mosquitoes:
Wear light colored clothing
Mosquitoes are attracted to dark clothing when they are looking for a place to land. Often, Mosquitoes make their approach from far away and dark clothing can often look like inviting foliage. Therefore, a natural Repellent is to wear lighter clothing.
Mosquitoes love Carbon Dioxide. Your body will release a significant amount of Carbon Dioxide while you are exercising or when you are hot, as in the summer. Candles and fires also emit Carbon Dioxide. Therefore, by staying cool and away from flames, you can naturally repel some mosquitoes. Staying clean is also, it turns out, a natural repellent - have a cold shower after exertion to remove sweat and cool down.
Avoid producing lactic acid
Mosquitoes love lactic acid. All human bodies produce lactic acid, especially during exercise and after you digest certain foods, such as milk. Therefore, find a list of those foods that create lactic acids and avoid them as a natural repellent. Also, remember to stay cool and (if possible) to limit extreme physical activity while outdoors.
Leave the perfume at home
Mosquitoes love the scent of sweet fragrances that women wear most often as hair spray and perfume. Therefore, by discarding these sweet smelling products, it is possible that you can avoid a few mosquitoes.
Use a natural repellent
There are many over the counter repellents that use essential oils instead of chemicals to repel mosquitoes. Or you can make your own homemade repellents using natural essential oils.
Because mosquitoes vary from region to region and by type, it may be beneficial for you to experiment with the tactics of the mosquito du jour in order to determine what methods are most effective with them.