Step 1 -- Start Early
You know the phrase the Early Bird Catches the Worm, This also applies to yard sales. I always start Yard saling Early - I'm out by 7am, Hitting the advertised sales and anything else I see on the way
Because if you get there while there setting up (or just as they open) on a thursday or friday (depending on the sals first day) you get to see everything, including the rare treasure say a copy of X-men #1 (1963) in a box marked Magazines 25cents each (has happened, a very poor copy, but has happened)... also there is a good chance if they have alot of items you want they'll sell you everthing at cheap so that they can close the sale and enjoy the weekend
Speaking of buying the whole garage Sale
You may think I'm nuts, But one of my favorite things to do early on the first day of the sale is, when I find a good garage sale I offer say $100 or $150 dollars for everything - it doesn't always work but maybe 2 out of 5 people take the money. Why because they figure, Hey I just got rid of al my crap - made $100 bucks and have the whole weekend free to spend it -- most people don't expect to make $100 or more on a garage sale or are only having the sale to get rid of unwanted crap w/o having to through useful items away.
But Won't the majority of that garage sale be CRAP
A: YES --- But you can pick out what you want or can EBAY and drop the remainder off at you local Goodwill or Salvation Army store and even get a tax write-off for te donation -- It's a win-win situation