Surveys can only be sent once per reward tier, so make sure that you are asking for everything you need in order to fulfill backers’ rewards. Don’t forget to also check the preview of a survey before sending, to double-check that nothing crucial is missing.
If you realize that you need information from your backers before the project ends, or after you've already sent the survey, you can message all backers in a given reward tier directly through your backer report. You can also message backers that fall into the specific search filters within your backer report (including backers that have or have not responded to their survey, backers with dropped or errored pledges, backers whose rewards you've marked as "Shipped," etc.).
Backers are emailed when you first send a survey, and they'll receive periodic reminder emails for two weeks. Additionally, when a backer logs in to Kickstarter they will see a notification banner at the top of their screen alerting them that they still need to complete a survey.