Give people adequate notice and deadlines. This practice should be completed every couple of days through the end of the survey. Deadline extensions of 24-48 hours should be planned into the schedule in advance as a precaution. If needed, a deadline extension announcement should be sent in the afternoon of the last day to non-responders. Also, if needed, send invitations to some or all of the people in your reserve database.
Incentives can also help increase response rates.
It is best to make them substantial and to introduce them right at the beginning so that the motivational value can be incorporated into the value proposition and applied as part of the overall frequency of communications during the survey process. As a last resort, if you still have not achieved a required sample size, you can employ a telephone survey facility to conduct a limited number of outbound interviews to complete your sample. There are also two things you can do with an undersize sample at the statistical analysis level. The other is to use analytic software such as SPSS Exact Tests, which analyzes and adjusts statistical reports to account for small sample sizes. Both of these techniques should be done by a research professional.