I think asking without to much gimmick is best. Just be transparent and open about wanting to them to refer others.

For instance, when I complete a clients project and we have built a great working relationship; I simply say if you know of anyone who you believe we'll be a great fit in assisting them on their next project, please keep us in mind.

I always ask those who I've established a relationship with on a personal and business level.

If I only did a quick project for you and we haven't really established anything or much... I don't really ask this. Honestly, if the work is great and the client was satisfied and everything ended great... they WILL refer you every chance they get.