Web-based social data analysis tools that rely on public discussion to produce hypotheses or explanations of the patterns and trends in data, rarely yield high-quality results in practice. Crowdsourcing offers an alternative approach in which an analyst pays workers to generate such explanations. Yet, asking workers with varying skills, backgrounds and motivations to simply "Explain why a chart is interesting" can result in irrelevant, unclear or speculative explanations of variable quality. To address these problems, we contribute seven strategies for improving the quality and diversity of worker-generated explanations. Our experiments show that using (S1) feature-oriented prompts, providing (S2) good examples, and including (S3) reference gathering, (S4) chart reading, and (S5) annotation subtasks increases the quality of responses by 28% for US workers and 196% for non-US workers.