YARD SIGNS DON'T VOTE: MAXIMIZE VOTER CONTACT EFFORTS
Every day thousands of people would come by Obama campaign offices around the country asking for yard signs. It was common on the Obama campaign to hear staffers utter the phrase "Yard signs don't vote." What this means is that people want signs to stick in their yard so they can feel like they're involved in the race, but their signs don't actually do anything. Especially on large campaigns where both candidates have high name recognition (or in the case of a Presidential campaign near universal name recognition,) a sign will not persuade voters.
Yard signs cost money ($1-$5 each) and it is a waste of campaign resources to give them out haphazardly. The Obama campaign developed a policy that you had to volunteer for the campaign in order to get a yard sign or donate $5 to the campaign. Many people were upset by this policy since they wanted to show their support for Obama, but tactically this was a smart idea to further engage supporters.
On smaller races, having yard signs can be an effective way to increase your name recognition. If very few people know you are running, this is a good way to get your name out there. It won't persuade voters to vote for you, it will merely tell them you exist. The only time yard signs persuade voters is when a low-information voter sees a sign on the lawn of someone whose political opinions they respect. "Oh, Linda's supporting Tom for School Board. She follows these things. I guess I should support him as well."
Yard signs can also be a very effective tool for messing with your opponent. It is common for a campaign to send a "tracker"to follow around their opponent. They will determine where he lives, where he works and the route he takes to work. Once you know the route your opponent takes, at 3am go out and line that path with your own yard signs. When they drive to work they will see dozens of your signs, overestimate your support and by the time they've arrived at their office you've already ruined their day. A grumpy and stressed out candidate will impede their staff's work as well.