Summarize:

Surveyors are looking for specific types of people, and while you won't qualify for every survey (if you're a 25-year-old healthy hipster and they're looking for a 60-year-old couch potato, you're just not going to make the cut). Many survey companies offer screener surveys when you register, which usually don't pay.  However, it's an important first step, and worth the little extra time it takes to fill these screening surveys out because the more demographic information the market research companies have on you, the more surveys they can send you. Remember, you will only be matched with surveys that you can actually contribute to, so if you leave all your screener information blank to get through it more quickly, you will receive fewer survey opportunities. Keep an eye out on both the site and your email regularly for survey opportunities. Some companies offer surveys more often than others—and you certainly don’t want to miss any of them!  Any individual survey site may only offer a couple of surveys to you in a given month. The more companies you sign up with, the more surveys you'll be able to complete. Set up a rule for your email so that any survey emails that come in are flagged, make noise when they arrive, and appear at the top of your list. Everything you do to make them stand out will help. Once you have lots of surveys available, you can be more selective and pick the ones that seem like the best use of your time. If time isn’t an issue, on the other hand, you can do them all. You're under no obligation to complete any surveys if you don't want to.
Be prepared. Fill out the surveys. Check often! Choose and complete the best surveys.