Timing outweighs everything else
The exact same question sent right after a purchase and sent a week later collects two very different numbers of answers. It isn't laziness: after a week the person has nothing specific left to say, and answering «eh, fine» feels pointless to them — so they don't.
The practical rule: if you're asking about an experience, ask the same day. If you're asking for a general opinion, timing matters less but the day of the week matters — Tuesday and Wednesday morning, not Friday afternoon.
The first question decides whether there'll be a second
Someone who has answered one question tends to finish: it's sunk cost pushing them on. So the first question has to be the one that's answered without thinking — a scale, a yes/no, a choice between three options. Never an open question, never the email, never «tell us about your experience».
The corollary is that the email goes last and stays optional. Asking for it first turns a survey into lead collection in the reader's eyes, and the reaction is to close the tab.
Say how long it takes, and be right
«Two minutes» in the invitation raises open rates. «Two minutes» followed by eleven questions lowers them forever, because next time that person won't believe you. A promise about length is a debt: only make it if you've measured.
If the progress bar shows ticks rather than a continuous fill, people see how many questions are left instead of a vague percentage. On short questionnaires that's the difference between looking nearly done and looking endless.
Who sends it matters as much as what it asks
A survey that arrives from a person with a first and last name gets more answers than the same survey sent from noreply@. It holds inside companies too, and it holds most when the request is awkward.
One last thing, obvious and almost always skipped: say what will happen to the answers. «We read every one and the next edition changes accordingly» costs nothing and moves the response rate more than any prize draw.
Send it immediately, open with a question people answer without thinking, promise a length you've measured, and sign it with a real name.