How many letter should you send to pre-foreclosure owner?

[Henley H.]

Their a a few pre-foreclosure homes I'm interested in and have mailed two letters to each of the homeowners without any response.  Is it inappropriate to send a third letter, or more?  Would history suggest it's even worth it to sen additional letters?  I haven't been able to find phone numbers for either owner and am no inclined to show up on their doorstep-