Today's DEET-based products are easy to use, have a nice "feel" on the skin, and offer light, pleasant scents. They come in aerosol and pump sprays, creams, lotions and even "dry" formulations. There are also pop-up towelette dispensers and individually-wrapped towelettes to take along wherever your adventures may lead. Some products have sustained release formulations. Others are designed for active folks who perspire a lot.
DEET-based products are sold in the U.S. in "big box" stores, like Wal-Mart, Target, Home Depot, Lowes and many others. You'll also find them in grocery, drug and sporting goods stores. If you are traveling abroad, buy your repellents here and take them with you. Some brands are sold online. If you buy online, be SURE to pick products that carry an EPA registration. That way, you know the product has been tested for safety and effectiveness.
DEET is the active ingredient in repellents it is the "gold standard" when it comes to effectiveness. See New England Journal of Medicine article on what repellents work .
Experts recommend using repellents registered by the U.S. Environmental Protection Agency. That way, you know the product will repel insects and ticks and that it has been evaluated for safety.
Consider the type of product (aerosol, towelettes, etc.) and how you will use it. Towelettes are more convenient to take on a hike, as an example. Concentration gives you some idea of how long you can expect the product to provide protection. The higher the concentration, the longer the bugs will stay away. Products last five or more years when stored in a cool, dry place and a family-sized container of spray should provide enough applications for more than one mosquito season.
If you'll be outside for 90 minutes, a 7-10% concentration of DEET in the product will work well. If you're off to fish where there are more bugs than fish or plan to safari in Africa, you'll want something closer to a 100% concentration. It will last 10-12 hours.
How much DEET shall I look for?