I've used yard signs for many years for my mortgage company, actually all my clients were based from yard signs + referrals they worked very well.
I've tried in the past to use them for a membership only site on how to make money online locally, but it didn't really work out.
I had a local number with a recorded message sending em over to the site, I did get many voicemails but not many hits to the site, I recommend having someone picking up the phone, that's how you are going to make most out of your signs from my previous experience.
If you want to go really blackhat, (this is how I did it to try the website thing) drive by Sat morning in heavy areas where you usually see them and pick them up from the competition, then with stencil lettering you can spray paint your message and put up 50 and test your results, or if you have good handwriting get a thick black permanent marker and write your sign.
**Warning you might get some phone calls from the sign owners** Just make sure you get a google voice number that you can change at any time.
Good Luck