I can't tell you how many times I would help a client and then for some odd reason they would go with a competitor because it was convenient, or their sister got in the business.
Don't give up on these folks! If you stay in touch with them they will come back. Especially if you're excellent at what you do. (and when they come back they usually are so ecstatic to be back doing business the right way they'll never go elsewhere again.
Just keep 'em in your mailing list. Keep adding value, and keep in touch.
If you aren't doing this you're losing more than you know.
Hope this helps,