1. Keep it simple
How simple? As simple as a first name and email. The more information you ask the less likely they are to fill out a card.
2. Avoid the phone number
People simply don't like to be bothered and don't want to give out their phone number.
3. Go loose, not perforated.
Guests don't want to interrupt the service with the sound of tearing the card.
4. Offer a gift (good).
This gift may be a mug with your church's logo on it. Just something that acts as an incentive to fill out and return the guest card.
5. Offer a donation (better)
For every guest card that is returned you can donate a dollar amount to a local ministry, say $5, for every guest card returned. That's a win-win.