Reminder postcards or telephone calls can be directed at non-responders to encourage participation. Postcards are one of the least expensive methods to boost response rates, but they are no help if the original survey has been lost or thrown away or if the database is poor. Use First Class mail instead of Third Class. Using postage stamps, rather than metered mail, further increases the likelihood of an envelope being opened. Enter respondents into a drawing for cash or prizes.
Drawings are usually most cost effective for large volume mailouts.
Affiliations with universities or community organizations lends credibility to any survey. The more legitimate a survey appears, the more likely receipients are to respond to it. Remail the initial package after 1 - 2 weeks. Pre-test your survey instrument before mailing. What seems obvious to the survey author may be unclear to the typical respondent.