Use a tape recorder in an interview. Ask the interviewee for permission to do this, as some people may be uncomfortable with it.
Encourage questionnaire respondents to reply by offering a prize. This can also be a small gift for everyone, or entry into a draw for a bigger prize. This can be used for unsolicited interviews, such as in consumer tests.
Surveys can also include other required actions, such as tasting new foods, watching videos or looking at pictures. Typically these actions are followed up with appropriate questions (often about preference).
Use a shaded background on questionnaires, leaving white only those areas that are to be written on. This makes it very easy to see where a response is required.