Qualitative surveys are emerging as a popular research method in software engineering (SE), particularly as many aspects of the field are increasingly socio-technical and thus concerned with the subtle, social, and often ambiguous issues that are not amenable to a simple quantitative survey. While many argue that qualitative surveys play a vital role amongst the diverse range of methods employed in SE there are a number of shortcomings that inhibits its use and value. First there is a lack of clarity as to what defines a qualitative survey and what features differentiate it from other methods. There is an absence of a clear set of principles and guidelines for its execution, and what does exist is very inconsistent and sometimes contradictory. These issues undermine the perceived reliability and rigour of this method. Researchers are unsure about how to ensure reliability and rigour when designing qualitative surveys and reviewers are unsure how these should be evaluated. In this paper, we present a systematic mapping study to identify how qualitative surveys have been employed in SE research to date. This paper proposes a set of principles, based on a multidisciplinary review of qualitative surveys and capturing some of the commonalities of the diffuse approaches found. These principles can be used by researchers when choosing whether to do a qualitative survey or not. They can then be used to design their study. The principles can also be used by editors and reviewers to judge the quality and rigour of qualitative surveys. It is hoped that this will result in more widespread use of the method and also more effective and evidence-based reviews of studies that use these methods in the future.