I do a combination of letters and postcards. My first is usually a letter, followed by two rounds of postcards. I mail everything about a week apart. Right now, I'm only marking to pre-foreclosure.
You can do a forum search for techniques. I know some people use yellow letters, greeting cards, postcards, etc. I haven't done yellow letters but I hear they are effective.
The response rate has slowed down lately. I think its because everyone is trying to do loan mods or waiting for Obama to bail them out. A lot of calls I get now are from failed loan mod people who have no other alternative.
I know in my area I'm competing with about 9 other direct mailers, so I try to get to the homeowners first. The areas I target produce about 500 new leads a week. Do anything you can to get them to open the envelope. Also, expect about a 10% non-deliverable rate. Remove those from your list before your next mailing to avoid paying extra postage.