Most unhappy users never say anything unless you ask them directly, which is exactly what an in-app survey is for. But a badly timed or overlong survey gets closed unread, so how you deploy it matters as much as what you ask.
Keep it short
Every question you add is a chance for someone to abandon the survey. Ask only what you'll actually act on. A single well-chosen question with an open-text follow-up usually beats a five-question grid nobody finishes. See Survey Question Types Reference for lighter-weight question types (Thumbs, Faces) that lower the effort to respond.
Time it to the moment
Trigger surveys near the action they're about, not on a fixed schedule for everyone. A feature survey works best right after someone uses that feature; a churn survey works best on the cancellation page itself, not days later by email. See Target and Trigger Your Survey for the trigger types available.
Target narrowly
A survey shown to your entire user base gets a worse response, and a less useful one, than the same survey shown to the specific segment it's meant for. Use segments, page targeting, and device targeting rather than defaulting to "everyone, everywhere."
Limit how often the same person sees it
Set a sensible stop condition (see Target and Trigger Your Survey) so the same user isn't asked repeatedly. The exception is a genuinely behavior-based trigger like rage click, where you want it to keep firing; see How to Trigger a Survey on Rage Click or Element Click for why that one's different.
Know when not to show one
Don't trigger a survey during a critical flow you don't want interrupted, like an in-progress checkout, unless the survey is specifically about that flow. Avoid surveying brand-new users in their first session before they've formed an opinion, and avoid stacking a survey on top of someone who just submitted a bug report through the same widget.
Act on what comes back
A survey that never turns into a decision isn't worth the interruption it causes. See What to Do with Your Survey Results for a concrete framework for turning responses into action, and Survey Responses and Results to review what's come in so far.
What's next