taropatch -
Go to Home Depot or a hardware store and find the "heavest gauge" rod they have that is "tempered" and "galvanized" steel and that is just slightly thicker than a coat hanger but still "bendable." I like to bend a loop in one end to make it easier to carry and for something to hold onto when poking into and out of the ground. About three foot in length should do the trick. You can round off the poking tip with a file so there are no sharp edges that will scratch the glass. Buy several of them! They're cheap! That way you will have back up's when you lose or leave one laying on the ground as I've done on a couple of ocassions.
What I do is just slowly poke it into the ground and feel (listen with your hands) for the "klink" sound/feeling, and you will know you've found/hit glass. Then carefully dig down with your shovel and find that rare "already broken before you got there" bottle that, had it not been broken, would have made your day, week, month and year. But don't dispair, keep searching, as there are plenty of unbroken, beautiful, rare, valuable bottles to be found in that sea of broken glass. After you've found your first "keeper" you'll be hooked for life!
Also take along some small digging tools like a garden trowel and a screwdriver for easing out those "Picasso's" that you will eventually stumble upon.
Good luck, and I hope you "dig" it.