Self-report by participants is one of the most common ways that researchers collect data, yet it is fraught with problems. Some worries for researchers are: “Will participants be honest or will they say what they think I want to hear?” “Will they understand the 
questions correctly?” “Will those who respond (as opposed to those who don’t respond) have unique ways of thinking so that my respondents do not represent everyone well?” and a BIG worry “Will they even fill out and return the questionnaire?”
One way to solve at least the latter 2 problems is to increase the response rate, and Edwards et al (2009 July 8) reviewed randomized trials to learn how to do just that!!
If you want to improve your questionnaire response rates, check it out! Here is Edwards et al.’s plain language summary as published in Cochrane Database of Systematic Reviews, where you can read the entire report.
Methods to increase response to postal and electronic questionnaires
Postal and electronic questionnaires are a relatively inexpensive way to collect information from people for research purposes. If people do not reply (so called ‘non-responders’), the research results will tend to be less accurate. This systematic review found several ways to increase response. People can be contacted before they are sent a postal questionnaire. Postal questionnaires can be sent by first class post or recorded delivery, and a stamped-return envelope can be provided. Questionnaires, letters and e-mails can be made more personal, and preferably kept short. Incentives can be offered, for example, a small amount of money with 
a postal questionnaire. One or more reminders can be sent with a copy of the questionnaire to people who do not reply.
Critical/reflective thinking: Imagine that you were asked to participate in a survey. Which of these strategies do you think would motivate or remind you to respond and why?
For more info read the full report: Methods to increase response to postal and electronic questionnaires
Discovering Your Inner Scientist 