Start off with a trip down to your local sign place. Get a sign of respectable size. I'm talking 30 feet wide by 4 feet high. Get your message up there in as few words possible, so they are as big as possible.
WE BUY HOUSES 555-1212
STOP FORECLOSURE 555-1212
Now, take this sign up to an overpass. We're looking for a high-visibility time and place. On the way to the mall on a Saturday morning. On the way to work on a Monday morning. Preferably, in a place that gets backups, so people have to look at it for a while.
If and when you get a call from code enforcement, play dumb. "I didn't know those guys put that there! Man, I'm sorry, I'll call them now to have it taken down!"
Then you wait a half hour to get a few more calls, and pack it up for the day.
"Bandit signs are a blessing and a curse. Personally, I hate to put them up. I hate getting out the ladder, I always feel like people are gonna throw stuff and honk and yell at me... But they get great results!" ...
Once again, they don't care that they are pissing off the people who live there! And, they don't care that they are breaking the law by putting illegal signs high on telephone poles and elsewhere in the public right of way!