Stop! Before you go out to buy sign supplies, go plot out your routes. These are the ways that shoppers will drive (or walk) to your garage sale. Unless your home is very isolated, there are likely at least three or four possible main routes that you should place signs on.
Unsure about where to put signs? Imagine that you are a first-time visitor to your neighborhood and choose locations accordingly. Good rules of thumb are placing one at every turn, occasionally on a long stretch of road, and near landmarks. Choose where your routes will begin and put larger signs in those spots.
It is incredibly important to keep track of what direction you need your arrows to point. Driving your routes is recommended, as it prevents mistakes of complacency -- sometimes you just can't remember which way to turn because it's become unconscious habit. As you plan, keep track of how many left, right, and straight arrows you'll need to make, as well as how many explanatory signs you want to hang.
All right, now you can go shopping for supplies! Garage sale signs come in all shapes, sizes, and qualities. How high-cost do you want your garage sale to be? Depending how many routes you've planned and how complicated they are, making high-quality signs can add up quickly.
If you want to go a cheap route, buy a fat Sharpie marker for a few dollars at a craft or office supply store and use it on something found around the house: paper plates, the insides of cereal boxes, or sides of cardboard boxes. (You can go to grocery stores and ask for cardboard boxes -- they usually have an ample supply on hand and are more than happy to give them away.) Even plain white paper will work but you should use a backing so it is not so flimsy.
The fat Sharpie method also works on probably the most economical but eye-catching technique -- the bright posterboard sheets. Posterboard comes in sizes much too large for every sign on your routes, so consider cutting them in half or even fourths to make plain arrows; using the same color for all your signs connects them as the same sale in a customer's mind.
The option of laminating your signs is only really worthwhile if you live in an area with severe weather. Hopefully, you've planned your garage sale for a weekend that is forecasted as sunny, but if you anticipate any rain, getting some cheap laminating paper could be a wise decision.
So what should your signs say? A few things are crucial: the date and time, the location (you can write down an approximate area if you don't want your address up), and what kinds of items are being sold. These are important for the larger, explanatory signs that will appear at the beginning of your routes and at key points along the way. Otherwise, your signs need merely be arrows pointing shoppers in the right direction.
To draw customers right to your driveway, hang an extra piece of posterboard or a sign similar to your other ones on a garbage can slightly in the street, so they know that they've reached their destination and can begin the treasure hunt.
And for advertising in the virtual world, try listing your garage sale at GarageSaleCow!